We prefer taking a mid-day break over sleeping in, but we are morning people and I love the light crowds first thing in the morning in every park. We also don't stay out very late any night because we have two toddlers. We find mid-day to be the most crowded time for the parks, except for fireworks times. Character pictures are important to my kids, and all parks have great photo opportunities when they first open, with shorter lines. Some people say that parks with EMH mornings are more crowded that day and should be avoided, so that is something for you to consider.
Here's what we do:
MK - We like the early morning - it's less crowded and there's lots of character photo opportunities. We leave either after lunch or after the parade. In the evening, we find a place outside the park to have dinner and watch the Wishes fireworks (if they're before 9 p.m.) so we don't have to navigate the crowds with the strollers. You can try to schedule a ADR for O'Hana and try to get a window seat (they pipe in the music of Wishes), or have dinner at Chef Mickey's or Concourse Steakhouse in the Contemporary, then go out to a balcony with benches to watch Wishes. A CM can tell you where to go to see them.
Epcot - we sleep in and go there later, since World Showcase doesn't open until 11 a.m. If DH wants to ride Mission Space or Test Track, he gets a Fastpass. It is worth it to stay there later for Illuminations anyway. IMHO, using an EMH morning for Epcot only truly benefits you if you want to ride TT or MS.
Animal Kingdom - go early - it gets hot there and it's only open until 5 p.m. Plus, the animal viewing is better in the mornings. Have lunch and leave, and in the evening, you can try eating somewhere along the Boardwalk if you want to see some of the Illuminations fireworks without going into the park, or somewhere near MK to see Wishes.
MGM - we like early morning or around dinner time. Mid-day seems the most crowded. Early morning there are great photo opportunities and the place is pretty empty. Later in the day there are good places for dinner and the Fantasmic show is great.
Sorry this was long, but hope it helps you out.