When Celestia awoke who knows how long later, she was quite refreshed and significantly less cranky.
Hmm, I didn't realize how badly I needed that nap until now. I don't think I've slept that good since I was a foal! Speaking of foals, now that I'm in the calm, well rested presence of mind to enjoy an awoken Luna... wait. Where's Luna?
It was at that moment that Celestia realized that in place of a cute, sleeping blue filly nuzzled against her there was just a cold blank space of nothing but air.
"Oh snap... LUNA!!!" Celestia screamed, full well forgetting her room is soundproofed and Luna would thus have never heard her anyways. Even if she had heard her sister screaming, there was no response or Luna coming back to her room. "Darn that magic inhibitor spell didn't last nearly long enough," Celestia panicked as she searched the palace with x-ray vision to see if she could locate her mischievous little sister.
Since the dawn of her very existence, Luna has been a bit of a prankster or, at the very least, good at messing with ponies through disappearing acts and jump scares. Oh Faust I hope she doesn't decide to jump scare me right now, I'm still waking up a little... Celestia mentally groaned.
Meanwhile, a certain little blue filly was spontaneously teleporting around the castle. She didn’t really know what she was doing, all she knew was that every time she did it, she was in a new place to explore with new ponies who seemed to really like her, and that made her happy.
Luna didn't quite understand what they were saying, but this "Celestia" name seemed familiar. Perhaps it was that massive white pony creature that fed her and cuddled her in the night prior. If that was the case, she decided she was happy to hear the name. In response, she giggled and smiled a smile that would have killed with cuteness if such things were possible. Then she decided to do her little trick again and see where it would take her.
Somewhere in the distance Luna heard a familiar voice, but disregarded it in leu of this strange pink pony in front of her. She wasn't as big as "Celestia," but she was still bigger than herself. Luna craned her neck up curiously to look at her before a very (happy?) high pitched squeal escaped the pink pony's lips followed up with Luna being swept up into her hooves and cuddled relentlessly.
Perhaps the initial shock scared Luna and she fought a bit, but when the pony's voice calmed down a bit and the snuggling became more gentle, Luna didn't mind it as much and cuddled back. After all, no one can resist good cuddles.
A few minutes later, this pony introduced herself as "Cadence" and asked Luna if she could repeat after her. Obviously, Luna could not and just stared at her blankly before mentally storing that information for later. Maybe she'd understand when she was older.
"Cadence" then began to play a game of peek-a-boo with little Lulu, which initially confused her. But then she grew to like this little game of pretending to disappear and Luna joined in, hiding under a pillow and then appearing out of it again. Just before Luna was gonna up her game and use her unpredictable little trick again to actually disappear, the big white pony entered the room and shouted what seemed to be her name. Luna at least responded to it, so it must be so.
"Luna! Oh thank goodness you're safe. Nothing's burned down yet and you aren't crying so I think all is well," Celestia said as she quickly replaced the magic inhibitor spell on Luna before she could disappear again. "I see you've met Cadence."
"Auntie Tia, when were you gonna tell me you have a foal?" Cadence asked with a raised eyebrow.
"I was going to tell you this morning, but then I couldn’t locate her. Though, it seems she's introduced herself to you already," Celestia smiled, glad to see them get along so well. I wonder how she would've reacted to Cadence as an adult? Perhaps they would still get along. They both love foals dearly. "Cadence, meet your aunt Princess Luna of the Night Skies," Celestia introduced with a tiny bit of fanfare.
"Aunt Luna?... I thought she'd be... older?" Cadence confusedly stated. "What happened?"
"My best guess is that Harmony has a strange sense of humor. I suppose, yes, Luna is no longer full of the bitterness and hate that caused her to become Nightmare Moon, but this is a bit... much," Celestia explained to her niece.
"I see..."
"The palace staff and I are already planning to discuss how to get her back into her adult form."
"Aaaaaw, can't she stay a filly a little longer pleeeaaaase? She's so precious and squishy at this stage," Cadence pouted a bit as she held up a smiling little moon princess who stuck her tongue out a little simply because "why not?".
Celestia couldn't deny that Luna was most likely the cutest little alicorn filly she'd ever seen. True, she may be a little biased since it was her sister, but her opinion still stood. "She can't stay a filly forever. We don't even know if she'll age in mortal years, alicorn years, or some strange mix of both! Hay, we don't know if she'll age at all! She may very well be a filly forever if we don't do something. So no, she needs to be returned to normal."
"How long could that take?" Cadence asked, hopeful that it would be long enough to enjoy time with her baby auntie.
"I don't know, days, weeks, months, years? We'll find a way..." Celestia guessed. "If you don't mind though, could you watch Luna for the day? I still have duties for the day (ugh) that I can't exactly reschedule. I'll make time to spend taking care of Luna myself sometime tomorrow."
Cadence withheld her excitement with all the grace and subtlety of a coffee mug crashing explosively on what is clearly not the kitchen counter in a formerly sleeping household at 4am.
"YESYESYESYESYESYESYES OF COURSE!!!!"
"heheh, glad to hear it..." Celestia said as she rubbed the glee out of her ears. Before she left, she said one more thing.
"Oh also, I suppose we'll need to baby-proof the castle for both Luna's sake and the sake of those who do not want their wing of the palace to spontaneously combust. Okay bye!"
"Wait what? How do you even baby-proof an entire castle?! Spontaneously combust?!?! How long does that magic blocking spell last again?! Tia!!!"