I'd recommend doing a quick search here for "Blizzard Beach" or "Typhoon Lagoon" or "Water park", as that topic is discussed often. Cliff notes version (we LOVE both parks and go often):
It's pretty much split 50/50. Really, they're both great.
BB has a cool area for kids ages 3-10 roughly called Ski Patrol. Has zip lines, iceberg/rope crossing thing, small kiddie slides. But TL has a new family tube ride, a fun kids area too. They have VERY different wave pools. Both are fantastic, but they're drastically different. TL has huge waves...if the kids aren't good swimmers, they just stay in the very shallow end, we've had our kids in that pool since they were literally 1 year old. BB has small, constant waves, like being in a choppy ocean. It's a lot of fun too, you can sit in a tube and bob up and down (until you get sea sick...

).
You won't go wrong either way. If I HAD to choose just one for a 4 and 7 year old, maybe BB because of Ski Patrol. But if you're there for as long as you're going for, then I'd definitely go to both.