My cousin lost three hens to a hawk during supervised playtime just last month.

That point about slow cookers running too hot is worth considering. Even models from the same brand can vary a lot in actual heat (it's a common problem, really). Checking with a kitchen thermometer could help figure out if yours is cooking hotter than it should.

Honestly a kiddie pool for chickens sounds like absolute chaos. Tbh just picturing them all trying to fit in there at once is kind of wild. Ngl keeping it clean has to be a nightmare, all that dirt and feather fluff. You must be hosing that thing out constantly, it seems like a way bigger mess than a regular spot in the ground.