One key element to being in the parks is the air conditioned shows. Kids enjoy most any show at Disney. Mid morning and mid afternoon at least once, find a show like A Bugs Life of Finding Nemo or such and just sit. Sometimes the kids nap through th show, but that's ok. The adults need that little rest time too.
I went with my nephew 4 and grand daughter 9 we stayed 12 hours. Take alot of water and hand towels to keep the kids from over heating
From personal experience i brought my 10 yr old daughter and 1 yr old son and we enjoyed ourselves all day.
Depends on the stamina of your kids. You can rent a buggy/stroller and we have managed 9am to 10pm with a 3 Yr old quite easily
Till 9 at night
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