I haven't tried a kiddy pool, but do have a big rubber tub that is similar and I have filled that a few different times for different groups of horses. most aren't interested in it. Some of them like to play in the water when I've got the hose running to fill the tub (I'd spray their legs and bellies and then they'd try to lay down on the side of the tub and roll in the mud I created while spraying their legs and bellies. Once in awhile someone will paw at the tub & spill the water out, but mostly once the tub is full and I and the hose are gone, the horses lose interest & the tub just sits there. Sometimes someone will drink from it, but the water in their water tubs is fresher so I end up dumping the rubber tub and putting it away again.
When I was a kid I had friends who had a big old pig named Arnold, and Arnold liked to play in the kiddy pool that us kids wanted to splash in. By the time that pig got done with the little pool, it wasn't fit for us. Eeewwww.