Unfortunately, you won't find everything you are looking for at one WDW resort.
As a resort, AKL fits the bill - zero entry pool, splash pad and playground, but transportation is not very convenient
BWI - beautiful resort, walk/boat/skyliner to two parks and a playground, but the pool is not conducive to small children, IMHO (no zero entry).
WL - smaller pool which is good for little ones, splash pad, but no playground (you can boat to Ft. W to play on the one there, though) and transportation isn't great
Poly or GF - great location and theme, zero entry pool, splash pads, convenient transportation (particularly Poly as you can walk to the TTC to catch the EP monorail), but no playground.
I would not recommend BC/YC (pool is sprawling and it has no splash pad nor playground) or CR (no playground, no splash pad and the pool is not good for littles)
I would consider your criteria in order of importance and go from there!