They both have great kids areas. However, in my opinion Typhoon Lagoon had more stuff to do for the whole family than BB did. When we went to BB, me and my youngest 2 children spent half the day waiting while DH and DS7 waited in line for Summit Plummit. I know it's a personal opinion, but we all seemed to enjoy TL more than BB. They have a great pool over by Crush N' Gusher where the little ones can play while older ones go on the ride. Plus, a lot of the slides are geared toward the whole family rather than just older family members. We also loved the lazy river at TL.
When it comes down to it, you should probably go with the theme you prefer more because I think they have done a great job at both parks in the kids areas, but my personal vote goes for TL. Hope this helps.