Hi! Im bringing my two little ones, (3 and 1) in August and I agree with many PP's:
1) take the car back the night before or in the morning.
2) 6 hours is a long time to take off from the parks. Even with my little ones we only plan on leaving for naps between 12:30-3:30. There is not a lot you can do in only 2 hours in the morning. I have heard great things about 'touring plans' etc, and while I think they are a decent guideline, it is unfair to say that the best touring time is before 11 and after 4:30… there is SO much more you can do between those two times like hit up a show (Aladdin, Muppets, Princess Faire etc), have a nice sit down lunch in a shady spot, do some of the slower, less busy rides, explore Pirate Lair Island, Explore the wilderness trail in DCA, etc. This is what we have typically done, and what we intend to do with our kids when we go, and we find it really helps to max out our park time, but still take a slower, more restful pace.
With our 3 and 1 year old we are planning:
Arrive for park opening
11:30am - lunch in park
12:30am - naps in hotel
3:30pm - back in park
5:00pm- quick dinner in park
8:00pm - leave park
8:30pm - in bed in hotel
However, if our 3 year old passes out in his stroller than we fully intend on staying in the park for the day.
Leaving the park can be a real pain, in my experience, and just adds a lot of walking.