Hi Nathan,
Thanks so much for visiting the Disney Parks Moms Panel with your question. Visiting a Disney Water Park is a must-do for my family on every trip to Walt Disney World! We enjoy splashing the day away. I love both
Disney's Typhoon Lagoon Water Park and
Disney's Blizzard Beach Water Park. Given your combination of ages and the requirements that you lay out, I would suggest Disney's Blizzard Beach Water Park for your family.
A large reason that I am suggesting Disney's Blizzard Beach Water Park is while both Disney Water Parks have areas for younger children as well as thrill rides for the adults and older children, Disney's Blizzard Beach Water Park also has Ski Patrol Training Camp, which is a fun area designed for older children and pre-teens. My guess it that your 7 and 9 year olds will love it. My 8-year-old daughter spent a lot of time there on our last trip. At Disney's Blizzard Beach Water Park you will also find Tike's Peak, filled with fun for your little ones under 48", slides that you can do solo for big thrills like Summit Plummet and Slush Gusher, and slides with medium-sized thrills such as Runoff Rapids. And your whole family could enjoy Teamboat Springs together - it is the world's longest group raft ride!
I hope that these thoughts are helpful for you, Nathan. No matter which you choose, I am sure that you will have a great time at a Disney Water Park. Please do not hesitate to ask if you have any further questions.