Our kids were 3 and 5 when we took them for their first visit this year. We went the first week of Jan, so weather was mild and not sucking the energy out of us, which I'm sure helps. We had 7 park days with a day off in the middle. We generally alternated days of rope drop/midday break/close down the park and later arrival/leave early days.
MK days were always our long days, as my kids loved it the best and would keep going as long as we were willing to stay. Epcot and AK, not so much. We did a table service meal almost everyday - which basically forced us to stop and take a break for an hour or so each day. That helped. And honestly, our rope drop to close days were easier. We left the parks about 1:00 (when crowds were pretty thick), went back to the hotel, crashed into a family coma for about 3 hours, then got up, had dinner within an hour of getting back to the park, then rode rides during thinner crowds for the rest of the night. The nap + timely dinner was pretty refreshing. Days when we didn't leave midday, we went to bed earlier - maybe 8:00?
My biggest surprise was that my husband and I needed (and took!) the naps and early bedtimes as much as the kids!