Hello Kevin and thanks for bringing your splash-tastic question to the Disney Parks Moms Panel!
Disney has two great water parks-
Disney's Typhoon Lagoon and
Disney's Blizzard Beach. Both are great options and, in all likelihood, both will be open during your stay. Each park has a fabulously-themed "kiddie" area for your three-year-old, with pint-sized slides, splash areas and sprinklers. Blizzard Beach actually has a few more attractions for the youngest swimmers, however, both have plenty of options for young children. Both parks also have tons of intermediate and more adventurous slides, pools and raft rides for your seven-year-old. Both parks also have relaxing Lazy Rivers.
My family actually prefers Typhoon Lagoon because of its MASSIVE wave pool. But, with children that are three and seven I'd choose Blizzard Beach, simply due to the fact that it has more options for younger children. Either way, you can't go wrong and I am sure you will have a great time.
I hope this helps with your planning and if you have more questions, come on back to the Panel- we love to help!
See ya real soon,
Anne