We don't plan full rest days because bored kids are more stressful for me. Instead, I'd plan lots of partial days Where you can either do the water park or hit a park. Start later, move slower, decide based on everyone's mood, weather, what you missed last time, etc.
Do you have a list of all the shows you want to see? Fantasmic! at HS is pur family favorite. Fireworks at MK and EP are must-dos as well. HS fireworks are really only for Star Wars fans. I love the tree awakenings at AK, but find ROL totally skippable.
Make sure you see both Lion King and Nemo at AK. We also like all of the shows at HS- Indiana Jones, DisJr, Frozen Sing-a-long, and BatB.
7dmt is the only ride I know for sure that separating kids would be an issue. Barnstormer is tight, don't know if I've ever seen three across on it. You can't let the 5 and 8 year old ride anything that the little one is too short for anyway, so be sure to lay that groundwork early in the trip.
Let us know if you have any specific show or ride questions!