At least two days at MK and one at AK.
If they like characters, then you must do one day at EPCOT! Meet Mickey, Minnie, and Goofy on a Character Spot Fastpass. Also the shortest lines for Daisy, Pluto, and Donald. Several princesses meet in various countries, and the line for Elsa and Anna is surprisingly short. The easier princess meal is at Akershus, and our favorite character meal anywhere on property is Garden Grill (Mickey, Pluto, Chip and Dale). The Seas with Nemo, Living with the Land boat ride, and Imagination pavilion are hits with that age as well.
HS is now mostly for the street entertainment. Disney Jr is great, but not much else for littles. Toy Story Midway Mania and Voyage of the Little Mermaid maybe. Beauty and the Beast is nice if your kids are good at sitting for a 30 minute show.
Take lots of pictures! When our kids were that age we thought the vacation was for them, but it was really for the adults to enjoy making memories together. The kids probably won't remember any of it, so focus on adults having fun too, even if it means splitting up and doing a couple things adults want to do without the kids. We often travel with large parties, usually 7-12 people! We've found that spending some time apart allows everyone to have a better trip!
I would plan 5 day tickets with 2 days at MK and one at each of the others. Schedule HS towards the end, maybe second to the last day. Decide during the trip whether you want to do a couple hours at HS or go back to EP or AK to see something you missed. You cannot see everything at a park in one day, so don't even try. With that age, it is important to relax, and go at a pace that you enjoy. When they want to spend an hour climbing on a playground, let them! Take pictures, sit on a bench, and have a drink.