While I agree with PP, I get that you are also thinking about FP rides like Winnnie the Pooh for less waiting for kids. I think that since MK has the most rides to try and get through, I would plan to spend 3 days there, and you can split up the rides you want to do and your FP. If you plan to arrive at the park early for RD, you can get on several rides with very little wait, and plan FP for late morning/early afternoon, or for later in the day if you think you will take afternoon breaks for the little ones to nap. If you are in property early EMH hr. is great for this as well, but if not avoid those parks, since there will already be people in ahead of you. Also, if you would like to do some character meet and greets, you can factor those in, and could use FP for some if you choose. (Kenny the Pirate's site has maps of the parks with where and when characters meet if you are looking for certain ones). Hopefully you might be doing some character meals to get some of those done more easily.
I would have your group list a few must do rides in each park, and you for your children, and then plan your day to include those, and whatever else you get is a bonus.(same with characters you want to meet). If you are spreading it over a few park days, you can split it up by lands at MK if you wanted, or certain things to conquer 1st day, and then others the other days. At MK , you also have the day and night parades, as well as Wishes and the projection show on the castle. If traveling with parents, maybe you do one nt. with kids in bed, and you two go do some adult rides, or take 4 yr. old by himself if he might stay up a little later.
At MK, as PP mentioned, PP and SDMT are top 2 to try and get FP for, with long lines all day, followed by the three mountains. if you go early as mentioned, you could start in Fantasyland, and go to PP first for example, and save that FP for something else. Since your little ones can't ride all of the rides, and your parents may not want to, you can also split up FP for different people. You could do PP and Pooh rides 1st thing, and then Dumbo and Small World with very little waits on the first 2 hrs. Philharmagic is also great for the whole family, and can be done anytime with little wait. The Barnstormer might be a good first coaster for your 4 yr. old as well. You could then FP the mountains for later for those who want to go on them, and do a late character brft. or grab CS lunch, or watch the afternoon parade, before leaving for a break if you want. If you come back in the evening, you could do Space Mt. And Buzz for your little ones if the wait is not long, and do the Country Bear Jamboree in Frontierland while doing Splash and Big Thunder(or on another day plan those). On a second day, you could start with Adventureland, and do Jungle Cruise, Pirates, and Aladdin first thing, and then FP the Enchanted Tales with Belle and Mermaid ride, and Haunted Mansion for adults. Just some ideas so you can see how to play around with these.
At Epcot and HS, FP are tiered, so you will have to pick one level one and two of level 2. At Epcot you will have to choose between Soarin, TT, and probably new Frozen ride and A&E meet and greet in tier 1. In Tier 2 , you can grab Nemo and Turtle Talk for kids. This is another park to plan two days for due to this. You can again go early and do one of the rides and then FP the other. TT also has single rider line if some want to do that. You might plan one day for Future World, and one for WS(more geared towards adults, but Frozen for kids, and Phineas and Ferb game or WS crafts might be good for little ones). At HS, same set up, Toy Story mania is great for everyone, so I would choose that for Tier 1 or ride at opening (lines will be crazy all day), and RnRC can be picked if not, or you can do single rider also. In Tier 2, you can choose ToT and then one of the smaller rides or shows. You can scout out show times for things you want to see as there are many throughout the day, and you can work into your plan. Your kids might be into Disney Jr. and B&B. The Little Mermaid and Muppets usually don't get long lines. If anyone is into Star Wars, then you want to plan time for The Launch Bay meet and greets, and doing Star Tours ride.
At AK, the Safari is a definite, and Lion King show is great for everyone, and FP allows you less wait. If anyone wants Everest, you can FP or do single rider. There is a cute kiddie ride section in Dinoland, along with the Boneyard play area, but no need for FP, unless adults want to do Dinosaur ride. There is also the Nemo show, and Kali River Rapids if weather is warm. There are nice trails everyone can enjoy, and night activities will be starting, if you might want to come back/ stay for those(or do second evening there on a down day from parks).
HTH! Let us know if you have more questions!
