One day is plenty at HS.
We typically only spend a half day there, but there is still plenty to fill out a leisurely day at the park, especially if you let the kids play in the Honey I Shrunk the kids play area for a while. We usually grab the FP+ for the things that are normally a longer wait: Toy Story Midway Mania, Star Tours (40inches required), and Tower of Terror (40inches required). The Great Movie Ride is also a nice ride with the kids, even if they don't fully appreciate many of the film scenes shown

The show lines are much faster, so we don't grab FP+ for them.
Animal Kingdom is a decent park, but again it's sort of small. You might consider checking it out as well though, since there are some fun things there are well. My kids love the petting zoo, the safari ride, The Bugs Life show, Everest, and the Rapids. With a 4-day pass, you might consider doing 2 days at the MK, 1 day at AK, and 1 day at HS. (If not, 3 days at MK and 1 day at HS, since a lot of people love MK the most and there is really a lot to see/do there with kiddos that age)
Have fun and enjoy your trip!