We've done trips with our kids when our son was 4 and daughter was 2 yrs, 4 months; 5 and 3 yrs 4 months; 5 1/2 and daughter's 4th birthday; and then we're going this summer for our son's 7th birthday, daughter will be going on 5 1/2.
Some redundancy here with what others have said:
- don't be afraid not to do something and to scale back;
- try to do mid-day breaks/naps/pool time
- try to get to the parks early, break (we usually headed back to the resort) for naps and even some pool time, then head back to the parks for the evening - you avoid the hottest and most crowded part of the day and everyone gets a chance to recharge;
- only have a few things that you are trying to do, then anything beyond that is a bonus (e.g., we'd have the couple FastPass rides we wanted to hit, then anything else we'd have time for is a bonus);
- again, a little redundant, but don't push too hard.
Additionally, what we've found works well for us is to plan the mornings more, and have more flexibility in the afternoons/evening. Also we've found planning more, but then not being afraid to drop plans and scale back.
Also, with dropping plans, etc., keep in mind that dining reservations are supposed to be cancelled a day out to avoid a charge. However, I think with a lot of the restaurants, if you cancel in person, they'll give you some more leeway. We've done it a couple times - one year our son got sick while in DHS (threw up all over the floor of the gift shop - that was fun

) and we had a dinner reservation at Sci-Fi later that day. I went to the hostess a few hours ahead and explained - they cancelled it no problem with no penalty. Also, was told at Whispering Canyon that if you cancel a reservation at a restaurant in person, they can do it without charging you whereas if you call Disney Dining to cancel day of, they are more likely to charge you the cancellation fee. So keep in mind if you have an in-park reservation for dinner but it's looking iffy, if you're in that park early in the day, you may be able to just go ask in person to cancel, and you'll have a good chance of being okay.