My experience, having had a toddler and a preschooler, is they love every single pool ever no matter how sad or tiny or small or large, and you should consider, as a parent, some of the following:
- ease of supervision. Some of the water play areas will require less supervision than a full pool
- budget. Everyone on the internet will spend your money for you, but maybe you have other uses for the cash?
- bar proximity, er, sorry, that's just me
Resorts with excellent water play areas, by which I mean they're basically wet playgrounds with mini-slides and no deep water: Grand Floridian, Poly, Wilderness Lodge, Caribbean Beach, Port Orleans French Quarter's will be open by then, Animal Kingdom Lodge. Beach Club/Yacht Club also has one, but it's part of a very large pool complex that would have made me nervous with littles. Depends on how good they are at staying near.
The Contemporary has a splash pad. It's okay. It wins on "supervision" and is the closest to Magic Kingdom, which is a big perk.
A lot of the pools have zero-entry areas, particularly the Value pools.
The big waterslides at Mods and Deluxes aren't going to matter, since they're one-at-a-time slides, and go into 3-4 foot water.