I'd go with BC for several reasons: easy walk/boat/skyliner to Epcot and DHS. BC lobby and grounds are beautiful. The entire Crescent Lake and Boardwalk area is beautiful, especially at night. Arguably the best pool at WDW. View of the Skyliner, which would probably be very cool to a 5 year old. Great food choices, especially when you factor in Epcot. Ice cream at both BC and at the Boardwalk. BC is extremely "stroller friendly" if your 5 year old has times where a stroller is needed. My entire family loves BC.
I stayed at Animal Kingdom Lodge one time, Savannah View Club Level. I didn't like it and would not stay there again. It just wasn't to my taste. It's very brown. Brown, brown, brown. The hotel, lobby, room and grounds seemed dark. Everything in the resort seemed to be a long walk. My teens would jokingly say "well, we're hiking to the pool now, see you tomorrow"... and they don't usually complain about walking. The animals were interesting, but I live near a couple of great zoos, so zebras/giraffes and such didn't really rock my world. As noted, park transportation is limited to the bus. A big negative. I did not like the food choices at AKL (I am not an adventurous eater). On that stay, we had split between Wilderness Lodge, which the whole family loved, and AKL. What a mistake. Everyone unanimously wished we'd just stayed at WL. I do realize that many people love AKL. My family did not like it at all.
A Poly / BC split stay sounds like a great trip. Also, on your check out / check in day, when you move resorts - if you're moving to BC, you have tons of stuff to do in the period of time that you're "homeless". You could go to the BC pool, wander around Crescent Lake, walk over to Epcot, or Skyliner over to DHS, then easily and quickly return to the resort when your new room is ready. Good luck and have fun planning!