BB definately. There really isn't anything at TL kiddie area for your 7 yr old. My dd's6 informed that TL's area was "for babies", and they aren't really very adventurous either. They might not let your 7 yr old use the kiddie things, a friend told me her very tall 5 yr old was not allowed to do the kiddie slides, the cm said he was to big.
BB has 2 area's: Tykes Peak for preschoolers, Ski Patrol for older kids. Your 3 1/2 can do either and the 7 yr old will be happy with Ski Patrol's slides and stuff. My dd's both loved the family raft ride, didn't find it scary at all. I found it very easy to keep track of all 4 kids at BB. I could sit at the side and watch them play in Ski Patrol. I had a harder time at TL.