BiB - Ariel in a Babyish Pink Onesie with locking zipper
These pajamas go by quite a few names: onesies, one-piece pajamas, footed pajamas, footie PJs, blanket sleepers, babygro (UK) to name a few. But it's all the same: a one-piece sleep suit, regularly with hood, sometimes with feet, and occasionaly with attached mittens. Mostly they close with a zipper in front, but sometimes with snaps or buttons, and rarely with a zipper in the back. This one is unique as far as I've seen, and has a side zipper, which makes it a bit of a contortionist act to get into it, but that only makes it more exciting if you like your clothing to be hard to take off again. I got this sleeper at eBay from an old German lady that I expect has long since stopped making such clothes.
To make the sleeper inescapable, I swapped the regular zipper pull with a locking one. These are made by YKK and you can replace zipper pulls on most regular-sized plastic coil zippers, which is a bit of a tricky procedure, but it worked for me with the 4 zippers I replaced until now. The zipper pull has a small keyhole, and once it is locked, you can still pull the zipper up, but not down. This makes for a great experience when pulling the zipper closed and with every step you know it won't be going down anymore.
This sleeper has another uncommon feature: the arms are so long that your hands totally disappear. That allowed me to tie a small rope around the end, making it impossible to stick your hands out. So it is almost the same as having attached mittens.
Of course I had locked the zipper before I gave the sleeper to Ariel, and she unsuspectingly put it on. That was a bit of a challenge: with the side zipper, the legs, the long arms and the hood it wasn't so easy to figure out where the legs were supposed to go. But with some help she got it right, and zipped herself up. Only then she found out that it wouldn't open anymore, and there was no way to get out. She didn't seem to mind too much though, because she loved the soft fabric and its warmth.