Most of the pools are really nice. The Value resorts do not have water slides, but your 5-yr-old may not care about that. One of the best pool areas is with the Beach Club and Yacht Club, near Epcot. It's like a mini water park and part of the bottom is sand instead of concrete. Animal Kingdom Lodge also has a nice pool that is zero-entry.
There are sandy beaches at the Polynesian, Grand Floridian and Wilderness Lodge, in the Deluxe category, and at Carribbean Beach and Coronado Springs in the Moderate category. No swimming or wading is allowed in any of the bodies of water, though. You can play in the sand on the shore, but don't go in the water.