Like the others said, you can't go wrong with any of the choices!
Not having stayed at the Poly, I will rely on what others have said, and my desire to stay there myself one day. The benefit of the Poly like @
DisneyDebNJ said is really great restaurants and proximity to the MK.
Beach Club has the phenomenal pool and slide, good restaurants (beaches and cream) with the benefit of being walking distance to Epcot and HS. (DD may like this part...) and just a short walk away is the Boardwalk for some great nighttime strolling and entertainment.
AKL, while a bit further to the parks, is still a gem. I think that this would be a better resort for your family when DS is a bit older and he can really enjoy seeing the animals. Right now he won't be that excited... Restaurants are exceptional and the pool is above average.
What it really comes down to is what works best for your family... is your DD the independent type who would like to explore E and HS a bit on her own, ot walk over to the BW? Do you expect to spend a lot of time at the pool? How important is proximity to MK or E or HS to your enjoyment....
No matter what your choice, you will have a great time. All 3 are beutiful resorts...and all three are exceptional experiences.