That is adorable that your daughter and Mimms bonded so well. I say let them sleep together every night

If you really want him to be happy in his create at night, maybe you could have your daughter have him take a few naps in it during the day so he gets use to being in there. It sounds like he is getting comfortable sleeping with her and probably doesn't understand why he not with her at night too. Mine sleep with me so I am only guessing. When mine wasn't fully potty trained, they wore a diaper at night, and now I put a potty pad down in my bedroom just in case they can't hold it. You will figure out what works best for your family, hang in there