Hi Kari! (love your name by the way...)
I sought the advice of my "water park expert" (my 9 year old son). He said both
Disney's Typhoon Lagoon Water Park and
Disney's Blizzard Beach Water Park are awesome but he prefers Disney's Typhoon Lagoon Water Park because there were more attractions that he was able to enjoy there during our last visit. He and I are not tall. At age 9, he is probably similar in height to your 7 year old. We found more height appropriate attractions for him at Disney's Typhoon Lagoon though he also had a GREAT time at Disney's Blizzard Beach Water Park - so you can't go wrong!
He says the must do attractions are: Castaway Creek (lazy river), Crush 'n' Gusher (minimum of 48" (122 cm) tall), Humunga Kowabunga (also 48"/122cm minimum), Storm Slides and the Typhoon Lagoon Surf Pool. The surf pool is quite amazing. It features 6-foot swells. My family and I typically hang toward the back of the surf pool in the shallower end. It's incredible and oodles of fun but you will want to stay close to each boy if they aren't strong swimmers. Start shallow and then work your way up.
Also - are you selecting only one water park based on time or because of budget? If budget is the primary reason, do you know that with the
water park tickets, if both water parks are open on the day of your visit, you may visit both? If you decide to try 2 in 1 day, be sure to be at the 1st at park opening so you can maximize your time.
Thanks so much for your visiting the Disney Parks Moms Panel. It was so nice to chat with you and I hope we hear from you again as you continue making plans for your magical Walt Disney World Resort Vacation!
Kari