M.F.
The ones for adults are usually too big for kids under 5 or 6 yrs old.
A bunch of preschoolers would have a ton of fun with a just a few inflatable wading pools, beach balls and some buckets. Or, check out the "typhoon lagoon" water park pool at Toys R Us. A friend got it for her daughter when she was 2 and the kids had a BLAST all summer. It's only $30.