I want to second PP who said to check park hours. You do not want to book Magic Kingdom on a Party day, especially if you think you'll kids will make it until fireworks.
Genie+ will be a great tool for Magic Kingdom, as you can book character meets as well as the less competitive kiddie rides. DO NOT STRESS over getting on the Peter Pan ride. Its popularity is entirely tied to nostalgia.
The other best way to meet characters is through character dining, but those reservations sell out months in advance. You can sign up for reservation finders through Mouse Dining (
https://mousedining.com/events/1) to try to snag last minute cancellations that tend to pop up the week of, but I wouldn't set my heart on it.
If you're looking to do three park days, you'll have to decide WHICH parks. Animal Kingdom has the safari, a petting zoo, and a fun playground. Epcot has the aquarium, space, international culture, and Frozen. Hollywood Studios is more of the "big kid" park, but they do have Toy Story Land and Star Wars, if your kids are into those.