This is asked quite a bit. Not being harsh, just saying you’ll find a ton of info here as it’s discussed a lot. Try searching “Typhoon Lagoon” or “Blizzard Beach” and you’ll find endless results.
Short answer is that both are terrific. It depends a lot on your 7 year old, what they like and their swimming ability. Both have great slides, both have high/thrilling ones. Both have lazy rivers, both have types of tube slides and such. BB has a neat kids area called Ski Patrol that has a zip line and iceberg crossing (hold onto ropes over your head and step across “iceburgs” without falling into the water). Great for a 7 year old. Then again, TL has some cool slides, such as Crush N Gusher that BB doesn’t have. It’s sort of a water slide roller coaster…though nothing super intense. The biggest difference between the two is the wave pool. BB is small, consistent waves. Like bobbing up and down relentlessly in the ocean. TL has massive, huge, gigantic waves that people can body surf on. Since it’s a zero entry pool, if your child isn’t the best swimmer, they can stand ankle deep in it and be fine…and still experience a bit of the waves (by time they get back that far, they’re pretty weak).
They’re both awesome, honestly you won’t go wrong either way.