My nephews had a lot of fun playing in the empty pool when I was painting it in 2003. Used 2-part pool epoxy paint and applied it with a paint roller. Very durable surface & pretty slick stuff.

I'd put my nephews into a large plastic bucket and they would slide from the shallow end, into the deep end. Bucket base was large enough, that it wouldn't tip over..so it was pretty safe & fun for them.

Playing ball inside it was also fun. Sitting in the deep end and bouncing/rolling the ball in the pool. The ball always returned back to you in the deep end.