Everypony took to sleeping shortly after Luna raised the moon. Celestia and Luna took to their own rooms while Cadance, Shining Armor, and Flurry Heart slept together in a guest room. Selene slept in her bedroom as well. Wanting to be with her family for the night, Luna decided to put aside her duty of guarding the dream realm. Ordinarily, she'd simply leave a bit later than usual and return later than her normal time in the morning, but a feeling she couldn't name nagged her to stay home tonight.
Only one pony wasn't enjoying her slumber. Selene enjoyed meeting her new extended family. Even Flurry Heart, as chatty as she was, proved not to be so bad. Yet, Selene couldn't sleep easy. She tossed and turned too many times until the agitation finally woke her up. She looked around her room before falling back onto her pillow. Did she have a nightmare? She couldn't remember it. Maybe she just wasn't sleepy.
Carefully, Selene climbed off her bed and pulled open one of her bedroom's large doors. She still hadn't quite become accustomed to having such a large room. She peeked into the hallway, glancing up and down. It was dark and the only ponies to be seen were the nighttime guards, standing firmly at their posts. The guardsponies of the castle frightened Selene more than much else. Not because they really were scary, but because they didn't do anything except stand about. At least, from Selene's point of view. It was unsettling.
She tip-toed out of the room, not bothering to close the door behind her. She didn't know which way to go, so she just started walking downward to one end, gazing all around the hall. The castle had such a different aura at nighttime. As she passed a window, not hidden by its curtains, she couldn't help staring at the moon for a short while. She was grateful, but still in huge disbelief she'd really been adopted by a princess. Though, she herself didn't want to be a princess. No, royalty was too busy for her liking. She'd seen that for herself. Luna never left for the dream realm before putting Selene to put and saying good night and she always returned before Selene woke up. But after breakfast, Luna would retire to her bedroom to sleep, leaving Selene in Celestia's or Sombra's care, whichever one of them had less to manage that day. Selene would go back to Luna's care in the late afternoon, when she woke, and most of her time would be spent with Selene, thanks to Celestia and Sombra picking up some of her evening duties. Still, that was soon to come to an end. Selene didn't know who would be in charge of her then. She just hoped whoever they were, they were nice.
Selene pulled herself away from the moon's glow and continued down the hallway. She paused at the doors and glanced back and forth between the two guards.
"Uh... can... can I go through?" she asked in an anxious whisper.
The guards shot each other a quick glance before looking back to Selene. "Do you need Princess Luna's attention?" one guardspony asked.
"No. I, uh, just wanted, uh, a snack?"
She didn't sound convincing in the slightest, but neither guard felt reason to reject. Besides, she didn't need to ask them to pass. They allowed her to go ahead, opening the doors for her, and she trotted ahead, trying not to look back at the guards as she passed through the doors.
Selene never traversed the castle without somepony escorting her, but she had yet to memorize where anything was, though she could recognize a room if she'd seen it before. She continued forward, no particular destination in mind, until she eventually did find herself in the kitchen after making a turn somewhere. Unsurprisingly, it was dark and empty of anypony. Selene was able to see it was spic-and-span. She went about trying to find the refrigerator, bumping into and tripping over counters, chairs, and one of the table legs, and knocking over a basket of something on the edge of the table. Despite the noise, nopony else came into the kitchen, so Selene assumed she wasn't disturbing anypony. Despite the dark limiting her vision, she did find the fridge and pulled open the door, letting the light pour out. The fridge was plentifully stocked, so much so, Selene felt anxious to take something without permission. Luna hadn't told her she couldn't, but she didn't feel confident she could. After mulling over it for several seconds, she closed the fridge without making a choice and stumbled her way out of the kitchen.
Exiting through the opposite of the kitchen she entered, she trotted down a different hall, still taking in the size and awe of her new home, until she came to a different room. She cautiously pushed open the door and peered inside. Instead of dark, this room was dimly lit, enough for Selene to see it was full of armor and weaponry. Likely not a room Luna wanted her to play around in. Still, her curiosity was piqued, so she stepped in and closed the door behind her. She approached one armored mannequin and stared at its golden armor. It was too high for her to reach and she couldn't fly. She looked around the room and spotted something that seemed like it could help: a broom. She grabbed it, returned to the mannequin, and as carefully as she could, stuck the broomstick beneath the helmet. She repeatedly tapped the helmet upwards until she finally managed to knock it off, tossing aside the broom so she could catch it. "Yes!" she squealed to herself. She set the helmet sideways on the floor and stuck her head in. Not at all to her surprise, it was too big a fit, but she managed to hold her head upright with it on anyway.
"I'm a guardspony!" she whispered in mock declaration. She picked up the broom with one hoof and held it over her small shoulder. "Nopony shall pass!" Putting on a comically serious face, she "marched" back and forth across the room. "Hup! Hup! One, two, three, four! I declare a pony war!" She pointed the broom menacingly at her shadow on the wall. "Halt! Who goes there?"
After she tired of pretending to be a guard, Selene slipped her head out of the helmet, set it next to the mannequin she'd taken it from, and left the room, remembering to close the door behind her. She looked up and down the hallway before running off, now feeling less nervous than she had before. The castle's halls still remained empty except for the guards on shift, who let her pass without trouble when she came by.
The next room Selene found herself in was the throne room. She'd only been in this room once: the day she came to the castle. The first sight that grabbed her attention were Celestia's and Luna's thrones. Unable to resist her playful instinct, she rushed over to the dais and climbed the staircase, but paused at the top. She stared at the regal pieces of furniture for several more moments before placing two hooves on Luna's throne. She hesitated a bit more, as if she was defying some unwritten rule, but she soon pulled herself up and sat on Luna's throne. It felt strange. She couldn't believe Luna really sat here. She fidgeted around, trying to make herself comfortable, but it seemed the throne was just too big for a little pony. Like everything else in the castle.
She set herself on the edge and hopped off Luna's throne, but instead of landing on her feet, she stepped on one of her hooves and slipped onto the dais before tumbling down the small staircase. She landed on the floor with a soft thud and a little bit of dizziness. She was mostly okay, but one of her front hooves was sore. She looked it over. The hoof was bleeding and had a light bruise. It wasn't a serious injury, but the sight of her blood scared her, and Selene began crying.
Instantly alert, the guardsponies standing outside the throne room rushed in. They surrounded her and, upon seeing she was hurt, tried to settle her down to let them take her to the infirmary.
"I want Luna! I want Luna!" Selene cried, holding her sore and still bleeding hoof.
Thankfully, the castle had an announcement system, something the royal sisters had installed some years back to prank each other with, but had become useful for more practical reasons later on. One of the guards picked up the system's microphone and hurriedly spoke into it.
"Princess Luna, your presence is requested in the throne room immediately!"
The announcement woke the whole family from their sleep and less than thirty seconds later, they were all in the throne room. The guards stood aside to allow Luna to approach Selene, who'd been made further upset by all the attention.
"Your Highness, she has injured herself," the guard explained. "She refused to allow us to take her to the infirmary."
Luna quickly spotted the injury. Selene's hoof had stopped bleeding, but Luna could see the bruise and that it still needed to be cleaned.
"Thank you, everypony. Please resume your posts. I will care for her." The guards bowed and did as instructed. Luna sat down and held Selene close with one of her wings until Selene calmed herself, which didn't take long once she felt Luna's warmth. She wiped her eyes with her unhurt front hoof and showed Luna the injured one.
"I can see." She laid down briefly to let Selene climb atop her back. "We shall take care of that." Luna turned to her family. "Thank you all, but you may return to bed. She will be alright."
The others nodded and returned to their rooms to sleep while Luna carried Selene to the infirmary.
"Luna, that stings! I don't like it!"
"It'll stop quickly. Be still," Luna whispered, holding Selene's hoof to prevent her from moving it. The sting from the rubbing alcohol did stop in a few seconds, and Luna dabbed the cut dry with a cotton ball before wiping the dry blood off with a moist wipe. She pulled out a box of large band-aids from a drawer and carefully stuck one on Selene's hoof. Selene's short treatment was done.
"How does it feel?" Luna asked.
"Better. Thank you, Luna."
"How did you injure yourself, Selene?"
Selene clammed up and looked away from Luna. She didn't want to admit she'd been doing something she wasn't supposed to. She didn't want Luna to scold her, or worse. She nervously looked back to Luna, who was patiently waiting for her answer and, to Selene's relief, didn't appear angry. Still, Selene was hestiant. She opened her mouth to speak, but shut it a second later and looked down, softly rubbing over her band-aid instead.
Luna pushed Selene's hoof away from her band-aid. "Selene, please look at me."
Selene raised her eyes until they made contact with Luna's line of sight. That sad face almost struck through Luna's heart like a stake.
"Tell me the truth. Whatever you say will not anger me."
Selene looked down again, but quickly brought her eyes back to Luna. Slowly, she began to confess. "I... I... went... to... I..."
Ever patient, Luna didn't rush her, yet that somehow seemed to be less helpful than it should've been. But eventually, Selene got it out.
"I was... I was playing. I couldn't sleep and I... I just wanted to explore the castle a bit. I was playing on your throne and... and I fell. I'm sorry, Luna. I'm really, really sorry!"
She became teary-eyed as she spoke, but she didn't have a chance to cry again. Almost as soon as she stopped talking, she found herself enveloped in that warm wing again.
"L-Luna? You're not mad?"
Luna shook her head. "I said I wouldn't be."
That was a first for Selene.
"Selene, I never forbade you to go about the castle on your lonesome. Nor did I give the guards orders to restrict you from passing."
"So, it's okay?" Selene asked.
"No," Luna replied. "You should not have played in the throne room, but you violated no restrictions. I cannot punish you for that."
"You didn't break any rules, so I have no reason to be upset with you," Luna clarified. "However, if you could not sleep, what prevented you from coming to me? You knew I did not travel to the dream realm tonight."
"I wanted to, but I didn't want to bother you," Selene admitted. "I didn't want to wake anypony up. And exploring the castle was kind of fun. I just wanted to try to find stuff by myself for once."
To Luna, "exploring" sounded more like an excuse to go snooping and get herself into some trouble when she wasn't likely to get caught, but in hindsight, it wasn't really a bad thing for her to learn to navigate the castle alone. She'd eventually need to because Luna and the others couldn't be her escort forever. At the same time, Luna was faced with a fact she already knew: Selene needed somepony to be there. When she adopted Selene, she made the choice to be that somepony.
"Let us return to slumber, Selene," Luna finally said. "You shall remain in my chambers tonight."
"You will sleep with me tonight in my room."
"Oh! Okay!" Selene jumped onto Luna's back again. Luna only smiled and the two exited the infirmary. Before they were even halfway to Luna's bedroom, Selene began to drift off.
"I must speak with Celestia about this," Luna mumbled to herself.