We love split stays, and my favorites are splits between an MK resort and an Epcot resort. So I'd vote for a split between Poly and either BC or BWI (YC is fine, but we prefer BC and BWI and both have been recently renovated.) It's always a tough choice between BC and BWI, but here are a few factors to consider:
BWI: Boardwalk views are right on the water and amazing. I think it's one of the best views at Disney and our entire family loves the energy at BWI. Most of the rooms have full balconies. The quiet pool is our favorite on property - in the middle of a courtyard with very little foot traffic and a super clean hot tub. The feature pool is fine - a little crowded for me and although the clown is creepy, the slide is fun. Love refilling mugs at the Belle Vue Lounge - so relaxing, no lines or waits and right off the elevators to the rooms. Love the Belle Vue Lounge in general!
BC: Love the renovated rooms - very clean lines, very simple and well done. The feature pool is wonderful and we've found several corners that are a little more quiet and serve as a nice base. Our kids range in age from 9 to 16 and are very strong swimmers, but when they were little, we just hopped in the pool with them and everyone stayed together for the most part. I'm a fan of the BC marketplace and love having a few quick service options, some grocery items and merchandise located in one spot. Also nice to have the beach area available.