Just to let you know, it is MOBBED for lunch. Like, wait an hour before you eat mobbed. I'd keep the reservation and keep checking back every day to see if a later time opens up. As it gets closer, people cancel trips, get sick, whatever, and you may get a later time. I say keep the time though because I went in January and it was FANTASTIC! And your kids will love it. You may also want to plan for a sit-down lunch that day at Plaza to give them something more substantial that will hold them until dinner. Maybe at around 2 pm.
August has been a tricky time to get reservations. We got one for earlier in our trip but are having trouble getting it for the second time (yes, we want to eat there twice. That's how good it was.) But like I said, keep trying. Also, Wishes tends to show later in August. Plus, since you're staying at Contemporary, it's easy to come back and catch a show.
Also, we'll be over at Bay Lake during part of your trip and then Beach Club.