To the relief of all involved, Luna did not accidentally combust Cadence's room into a giant pile of ash and the magic inhibitor spell held until it was Celestia's turn to watch Luna.
"I can't thank you enough for watching Luna for me," Celestia said as she picked up Luna from Cadence's wing of the castle.
Cadence was just finishing up putting away some toys as she responded, "Oh, it was no trouble at all! Luna is an absolute angel. We had a lot of fun today!"
With the last of the toys put away and a sleepy Luna in tow, Celestia thanked Cadence again and bid her goodnight. On the way back to her room, Celestia noticed that Luna had fallen sound asleep on her back. Oh, you had a big day today didn't you? Hopefully you'll be as good for me tomorrow as you were for Cadence.
-Later that night-
"Waaaaaaaaaaaaaah!!! Waaaaaaaaah!!!! WAAAAAAAAAAH!!!!!"
"Uuuuugh, Luna please GO TO SLEEP!!!" Celestia grumbled as Luna just kept crying. "What's wrong?! Ugh, we've gotta teach you how to talk ASAP..." Celestia mused as she tried to figure out what Luna wanted. Bottle? -swats it away- No... Cuddles? -still fussy- No... -sigh- I really hope it isn't what I think it is... -sniff- UGH! Those hopes were ill placed, yup.
"Waaaaaaaah!!! Waaaaaah!!!!"
"Yes yes, I know Luna, I hear you. Let's just get this over with..." Celestia said as she donned a hazmat suit fitted with a respirator. As Celestia pulled out some diapers and wipes the castle staff had brought earlier that morning, Luna just sat on the changing table, still crying. Once Celestia began changing Luna's diaper, she calmed down quite a bit and even seemed amused by Celestia's silly outfit and started kicking her legs around.
"Luna, hold still! Getting this diaper on you is hard enough as it is!" Celestia pleaded as she accidentally put the diaper on Luna backwards. "... whatever, that shouldn't be a problem." Celestia laid Luna back down to sleep in the crib the castle staff had provided (I swear, these are the most efficient ponies I've had in this castle ever!) and watched Luna finally fall back asleep. "-sigh- Thank goodness that's over with."
-Two hours later-
"Waaaaaaaah!!! Waaaaaaah!!!!"
"You have got to be kidding me..."
This cycle repeated about every two hours or so, alternating between bottle and diaper (or both).
"Waaah, waaah, waaaah!!"
"LUNA I SWEAR IF YOU POOP THAT DIAPER ONE MORE TIME I- oh, it's clean..." Celestia started before stopping at a stink free sniff test.
"Waaah!" Luna fussed.
"Are you hungry?"
Luna seems uninterested in the bottle.
"Your diaper is clean, you're fed, ummm..."
Luna isn't exactly crying now, but she is looking up at Celestia with those big baby doll eyes. She then reaches out for Celestia.
"Do you... need me to hold you?" Celestia asked as she picked her up and held her in her forelegs. Luna snuggled herself right against Celestia's soft chest. "Huh, well okay."
Celestia held Luna and walked around (as best as she could with only three available legs) until Luna started to fall back asleep. But, as soon as she tried to place Luna back in her crib, she started fussing again reaching for Celestia to hold her again. She toted Luna around and repeated this cycle about four more times before realizing it was hopeless, Luna was not going back to sleep in that crib.
"-sigh- Do you want to sleep with me tonight?" Celestia asked. She wasn't expecting any sort of answer, but Luna seemed happy at the proposition. So, she brought Luna with her back to her bed and they snuggled together until Luna finally fell asleep (and stayed asleep). Celestia's internal clock told her there were only three or four hours left in the night, but it was a good thing she'd cleared her schedule for the next day. Perhaps I can sleep in tomorrow to make up for tonight...
The next morning, Celestia groggily got up to raise the sun. She was just about to go back to sleep when she got the nagging feeling she was forgetting something...
Was there somewhere I was supposed to be today? No no, I cleared my schedule for the day... Oh no... Celestia had woken up just enough now and could see with the light of the sun the MASSIVE mess in her room. But most notable was the prominent lack of a little blue alicorn filly.
"Oh BUCK!!! Forget baby-proofing the castle, I NEED TO BABY-PROOF MY ROOM!!! LUNA WHERE ARE YOU?!" Celestia shouted. As soon as I find that little stinker, I'm having a magic inhibitor ring made in HER SIZE!!! We've never needed one that small before... I mean, what are the chances a magical threat faces Equestria who is the size of a filly?
Celestia searched Cadence's room first to see if Luna somehow decided to go there.
"CADENCE!!! IS LUNA WITH YOU?!"
"GAAAH!!" Cadence yelped as she unceremoniously fell out of the bed. "Uuuugh, good morning to you too auntie Tia. Why are you shouting at me at..." -looks at her clock- "THE LITERAL CRACK OF DAWN?!?!?"
"I guess that answers my question..." Celestia noted as she saw precisely zero mischievous blue alicorn fillies playing in Cadence's room.
"She's not with you?"
"Clearly..."
"... So where is she?"
"DO I LOOK LIKE I KNOW WHAT I'M DOING?!?! I DON'T KNOW!!!" Celestia screamed. Just then, an audible explosion could be heard in the distance. "This room... is not soundproof, is it?..."
"No..."
"Darn it... I'll have some explaining to do later. For right now though, I think I know where Luna is..."
In the ballroom, Luna was having fun playing with her magic and creating all sorts of strange plants and creatures, some that had gone extinct long ago, and some still around today. Luna also made a small star in the room that then exploded into a plethora of fine glitter (I mean, usually these things are put in the sky, but not today).
"Luna?! What happened in here?!" Celestia asked, as if Luna could actually answer that question. Just then, a chicken flew past her and into the palace to who knows where. "Gah!!! That was you?!" Celestia screamed as she tried to cope with the fact that Luna was the creator of the most vile, most horrible, most terrifying birds to ever grace the planet Equus. We'll deal with that later...
"Okay come on Luna," Celestia said, once again swooping her up and inhibiting her magic. "We're getting you fitted for an inhibitor ring."
Breaking News
Long forgotten extinct creatures have been spotted near Canterlot Palace! It's uncertain where they came from or how long they'd been hiding, but when asked about the new sightings, Her Royal Highness Princess Celestia said, "No comment."
In other news, inhibitor rings now come in foal's sizes! Perfect for that one overactive magical unicorn foal that needs a little help controlling their magical disaster causing surges. *Not available for public purchase, rules and regulations apply