Both parks are great, my family is hooked on them and we now spend more time at the water parks than the theme parks. Both parks have numerous slides/rides for each age group.
TL-I find it very relaxing with a tropical feel, so I try to go to this one when I feel I need the relaxation or more down time. Still lots of slides and fun though. If your kids like wave pools, this is the one to go to. I think this is a great park for families. It also seems more shaded to me than BB. It is my personal favorite.
BB-If your kids are really into more thrill rides/coaster types then they might like the bigger variety of slides at BB. Has a great pre-teen area good for kids your age. My 7yo son loved it. Family raft ride is way better and longer at this park than TL.
We always get at the water parks when they open. If you're staying onsite you can arrive 1 hour earlier than other guests and will feel like you have the entire park to yourself. Do all of the slides early, then just relax in the lazy river in the afternoon when it starts getting busy and lines for slides start forming! We arrive by 9am and leave around 2-3pm when it starts getting crowded.
One fun thing for kids to do in the morning is go on the lazy river and look for all of the change that people have lost/thrown into the water. My kids usually end up with $5-$15 each day in found change. My son puts on goggles and has a great time. Most change can be found in the early morning, near the lazy river entry points, and by bridges.
You can't go wrong with either park, they are both great. If I had to chose just one I would probably choose TL. But if you can, go to both, a day spent relaxing in cool water is much better than a hot day at a crowded theme park (at least in my opinion)!