I agree on the BB reco as well. They have a 'Junior Ski Patrol' area which is specifically designed for 5 to 12 year olds, so your 6 year old would probably love it. We were there last June with 3 kids....ds-9, dd-5, and ds-2....and all 3 of them preferred BB over TL. The oldest actually went on all but the very tallest water slides, and even the 5 year old (who had just turned 5 and couldn't really even swim) found several slides outside of the Jr. Ski Patrol area that she liked as well. BB also has an absolutely great family raft ride that was a lot of fun.....us parents took turns as it would have been a little too much for our then 2 year old, but we're looking forward to returning to that this year when he'll be 3 (almost 4.)
TL does have a better wave pool.....that was a hit with our 5 year old but not as much as a thrill for the others. TL is also nicer landscaped (but that doesn't matter to most kids!)
My general understanding is that the waterparks tend to be most crowded on the weekends when the locals aren't working, so avoid those days. I've also heard that early in the week is better than later because a lot of people put off the water park until the end of their visit.
The entire Summer is pretty hot. We've found the resort pools are great for a mid-day cool off break. A visit to a water park is more of an event for a 1/2 day or more. If you're going to go to a water park, you'd want to go first thing in the morning or arrive in late afternoon. Mid-day would be peak time, and some of the water parks reach capacity and turn people away at that time.
Definitely visit one though. It was a favorite part of our kids' vacation and is now on the list of things they want to do first.
