Then absolutely go for the 10 days and split the 2!!!
I know this is not common- but I am not a huge Stormalong Bay fan, or of the theming at BC for kids.. I actually think my own kiddo would enjoy the seemingly unpopular pool at BWI more. Plus, in December, don't let the pools dictate where you stay, or at least not in the same regards that they would have in the warmer months...that's what helped with my decision between CR and Poly anyway.
We are staying at the Contemporary this December and I am so looking forward to visiting WL!!! I'm hoping we will have time to rent bikes and ride over to FW to check out the decorated campsites, but not positive on that. If you split your stay between Poly and WL you will definitely be able to do that though!
My avatar is of my delicious drink at Poly while sitting in the hot tub, I don't randomly take beverage photos I just cropped myself out.lol It's a beautiful view from that spot, and even if it is chilly you can hang out there for a bit...you will absolutely fall in love with it! I haven't been to WL yet, only have looked at photos, the fireplaces around the lodge look gorgeous... take lots of photos to share with us!