I suggest a combination of everyone upthread's advice. Every suggestion has merit--send someone to get the fastpasses, don't make the kids go every time, adjust the touring plan to your tastes and timeframe, etc.
I would just add that it also may depend on how active/fit your kids are whether or not it's too much for them. Do they run around a lot at home, ride bikes, like to take walks, hikes, etc. or are they more couch potato types? I know some folks try to get in shape for WDW before they go, so if you have time, maybe you could start a little walking training before you go--lol, just walking after dinner every day or something will help...oh, and also, the heat may be a factor in how tired they get. Make sure they drink a lot if it's hot.
Also, involve them in the plan--invest them in it--so they know there's a method to the madness, not just you randomly saying, no we can't go on that ride there, we have to run across the park for no apparent reason...lol.