You're probably a little too far to go back to your hotel every day.
Some suggestions:
If MK is open all day (i.e. from 9:00-10:00) then you may want to take a break that day. Get there first thing in the morning, plan to leve about 1-2pm, then come back around 5.
AK is a small park and can easily be seen ina few hours (no need for break)
MGM is somewhat large, but has a lot of sit down shows so there's plenty of time to rest.
EPCOT is large, but lots of large buildings where you can go in and relax.
Make sure you get (or bring) a stroller for the 4-year old. In some of the parks, you can get a double stroller, which you could also use to give the 6-year-old a break (assuing their not too large).