Amy,
There are a number of activities that everyone can enjoy together. Here are just a few ideas:
Have Mickey Ears ice cream while watching a parade.
Let the kids play at the themed playgrounds,
Honey I Shrunk the Kids at Hollywood Studios and The Boneyard at Animal Kingdom.
Go on the Kilimanjaro safari ride at the Animal Kingdom.
Pose for pictures with favorite characters (older kids might want to start an autograph collection).
Take the raft to Tom Sawyer’s Island and let the older kids explore while the younger kids play.
Ride Walt’s train around the park .
Ride the clam-mobiles in the Nemo and Friends attraction at Epcot, then the whole family can share a laugh at Turtle Talk with Crush.
Since most rides will depend on a child’s level of tolerance for the dark, movement and such, you’ll have to gear the rides to each child’s preferences.
For an August vacation, I would suggest a mid-day rest back at your hotel. A swim and/or nap will refresh the whole family. Don’t forget sunscreen and to stay hydrated. While touring, your youngest children may benefit from some quiet times at the Baby Care Centers. These quiet facilities offer time out of the sun and a place to unwind. Have a mini-flashlight for the younger ones to carry while on “dark” rides such as Peter Pan.
Don’t try to see and do everything. Slow down and enjoy your magical vacation.