You're getting the best of both worlds in Poly and BC. Those are our 2 favorite resorts. They are 2 entirely different experiences, though. With Poly, you'll be more "in the action" by being in the monorail loop and having a view of the Castle right from the Poly beach/pool and possibly your room. With the BC, you'll have very short walks to both Epcot and the Boardwalk. You can walk to dinner each night and choose from 100's of places, including Epcot countries, BC, YC, Boardwalk. You can walk or boat to MGM also. You can rent surrey bikes and it's the best experience in this area because it's a beautiful loop around the water and you can watch Illuminations at night as you bike around.
Either pool would be great for your kids' ages. Our kids are 3 yrs old and BC is their favorite pool because of the sandy bottom pool area -- it keeps them busy for hours.