You can't beat the theming at Disney's water parks. My kids are 9 & 11 and love both TL and BB.
We go to TL on a day that we just feel like relaxing a bit more, do all the slides first thing in the morning, then spend the afternoon in Castaway Creek and the wave pool. (much better than BB's) The new coaster and swimming with the "sharks" are also good selling points.
We follow the same plan for BB, all slides before it gets busy, then lunch, relaxing and wait in line for our favourite rides again. Teamboat Springs is the best family raft ride we have ever been on. BB is a bit more adventurous, the water slides are longer and more varied.
All depends what your children like to do more.