Wow, your in a very similar spot to us back in '01!
Youngest DD was 6, DS was 8, and oldest DD was 9, but turned 10 while we were there.
Yep, have the kiddos involved with the planning from the get-go. It gives them some "ownership".
We also took a backpack with snacks, drinks, panchos brought from home, etc., into the parks. It made for a quick and easy way to take care of all those little thirsty, hungry times for them.
And, as was stated above, don't worry about trying to "do it all". Have a general plan, but we tended/tend to do a bunch on the fly, as well. But, mostly, try to take it easy and enjoy all the details and Magical memories!
BTW...
Congrats on the trip!
