"Heh, that wasn't so hard."
A cocky smirk found its way onto Glitter's face as he telekinetically shut the window he had just slipped through. His teal eyes darted back and forth as he quickly scanned the moonlit hall; he let slip a low chuckle. Empty, just as planned. This is almost getting to be too easy the Meowstic thought to himself as he began tiptoeing down the corridor.
The sneaky psychic Pokemon's movements, silent as they were, were being punctuated by a quiet crinkle. As he walked, the thick, heavy, heart-patterned Pikachu pull-up he was wearing swayed back and forth between his legs. The pink and purple garment had already had the majority of the day's drinks put into it, making the front, bottom, and even part of the back puffy and squishy. Additionally, the previous night's dinner was currently bulging out the back and bottom, having been deposited there that morning. Aside from making it harder for the Meowstic to walk (let alone sneak), the diaper also smelled terrible; it was clearly time for it to meet a trashcan.
C'mon, just a little bit longer Glitter urged in his head as he glanced down at the heavily-used padding. He then put a paw to his crotch. As if it wasn't already bad enough to be sneaking around a museum at night with a dirty diaper on, the cat Pokemon was also trying to do it with a full bladder, not to mention bowels that were about ready to burst.
Of course, it wasn't like this wasn't his choice. Glitter preferred being in this predicament whenever he was pulling a job; it made it feel so much more dangerous, so much more fun. Just like the fact that he had his teal-streaked white hair done up in a ponytail held by a pink hairband (with a matching band binding his tails), or how he was wearing nothing else but his usual heart-patterened pink socks and armbands, rather than something less conspicuous. Sure, anyone could pull a successful heist in a black bodysuit, but how many could do one dressed like a sissy toddler?
"Ah, here we are," the psychic-type murmured as he reached a set of double doors marked "Special Exhibits". He opened one of the doors just wide enough for him to slip inside, then quickly closed it from the other side. Masterful he mentally praised himself. 
Hanging above his head was a banner proclaiming "Kalos Culture! April 5th through May 4th!" It appeared the "C" in "Culture" had been added on afterwards, as it was clearly, upon inspection, covering up a "K". Glitter snorted.
Turning his attention to the room at large, the kitty surveyed the area. "It's gotta be around here somewhere..." Spying something promising at the far end of the room, be began stalking across the gallery as quickly and quietly as he could manage. "Bingo," he nearly blurted out a little too loudly when he reached a display labeled "Early Child Care".
A tableau depicting a turn-of-the-century nursery stood before Glitter. A wax Gardevoir was tending to a host of baby Pokemon, all frozen in the middle of various activities: up in the front, a Happiny was biting her pacifier angrily as she chased a mischeivous Mime Jr. making off with her Oval Stone; a Cleffa and Igglybuff were peacefully playing with blocks towards the middle; off in the back, hiding behind some cribs filled with other baby Pokemon, a Riolu was squatting, a flushed look on its waxy face as it finished up filling its diaper. That last one was Glitter's favorite.
Adjacent to this scene were various relics from this era, with informational cards explaining each item's significance. The sneaky 'mon quickly scanned through these, making his way through the exhibit until he found what he was looking for. "Geez, 'bout time," he muttered as he waddled up to a display detailing the invention of the disposable diaper.
Resting in a thick, transparent plastic case was an unopened package of disposable baby diapers. It, according to the nearby informational panel, was one of the first packs of disposable diapers ever made, at least in the Kalos region.
A look of immense concentration filled Glitter's face as he psychically probed the display for any kind of alarm. Upon finding none, he smiled. "Man, these guys obviously don't know how to protect their shit. This is too easy! Like taking...well, diapers from a baby, I guess." He shrugged. "Whatever." After mentally preparing himself, he unsheathed his claws, lifted the top off of the case, cut a slit into the plastic packaging, pulled out a diaper, and replaced the lid, all in quick succession. 
Perfect he thought as he clutched his crinkly prize close to his body. "Just one for now; I can get the rest afterward," he whispered with a smile as he looked down at his ill-gotten gain. The garment was smooth and plasticy, relatively thin, and a rather bland shade of white. "Hmm, doesn't get much more basic than this," he mused as he opened up the diaper. Though it had been made for baby Pokemon, the young thief was fairly certain that he was just petite enough to squeeze into it. He folded the padding back up, looked around, then headed for one of the empty cribs in the nearby display.
"Let's do this," he quietly said with a big grin. He unfolded the diaper and laid it out on the crib's mattress, then stood over it, orienting himself so that he could put the padding on properly. He unsheathed his claws again, lightly running them across the length of his heavily used pull-ups; the soggy garment slowly started to drip from the carefully cut slits. He then telekinetically lifted the baby diaper up under him, threaded his tails through the hole in the back, and put it on, taping it up snugly over the soiled pull-up he was wearing. This new layer of padding hugged his already-used diaper closer to him, pressing its wet puffiness and warm mushiness up against him in pleasant ways.
"Now for the best part," Glitter said as he spread his legs and squatted. He sighed in relief as he relaxed his bladder, letting loose the torrent of pee he had been holding back all evening. The warm stream flowed out of him and into his soaked pull-up before leaking into his outer diaper. The pristine padding slowly but surely started to puff out in the front and bottom, growing slightly heavier as it soaked up the kitty's cascade.
At the same time, the Meowstic's bowels started pushing, managing to slowly squeeze a solid log into the already sizeable mess in his pants with much effort. Grunting and straining, he kept pushing, eventually forcing out a softer flow into the seat of his pull-up, causing both garments to bulge out and sag from the heavy mess. All the while he kept peeing, eventually causing the back of the tape-up diaper to grow damp and yellowed.
When he finally finished, the pychic Pokemon exhaled and stood up straight, or at least, as straight as he could, given the immense bulk between his legs. "Heh, not bad, for something so old," he chuckled as he reached his paw down and pressed it against the crinkly padding. The diaper, though still relatively thin, had become puffy and tight enough to press the messy warmth of the pull-up beneath it firmly against him in the most comfortable way. As he poked and rubbed various parts of the thoroughly used padding, a stiff lump slowly formed in the front of his diaper, tenting out into a rock-hard bulge.

Glitter corrected himself. "Okay, now for the best part."