Denise,
First off,
absolutely you should plan out your dinners. Since there is a free dining plan promotion, the table service restaurants will be abnormally booked up during this time, so having some advance dining reservations will make your trip more pleasant. I would especially focus on a fun dinner on the 14th that all ages like at Epcot, like perhaps Biergarten, Le Cellier, or Les Chefs de France.
As for the water park, with your group, I would suggest Blizzard Beach. I think that your kids will find the water "thrill rides" there more exciting.
For parks, here is where I would go;
- September 9 - Magic Kingdom
- September 10 - Epcot
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September 11 - Hollywood Studios
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September 12 - Magic Kingdom
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September 13 - Hollywood Studios
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September 14 - Epcot
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September 15 - Animal Kingdom
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September 16 - Blizzard Beach
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September 17 - Magic Kingdom
Write me back if you have the park hopper, or if you are looking for multiple days at water parks!
Have a magical vacation!