I've always been a park hopper fan, but on my recent trip, I had park hoppers and did not use them. I did one park each day. I didn't exactly design it that way, but that is how it worked out. I'm not sure how much the difference in cost is for park hoppers, but it is not a necessity to have them.
It gives you a little less flexibility, but with a little planning, works out fine.
I have a DS11 and DD9.... for that age group, the highlights would be depending on their spirit of adventure:
MGM...Lights, Motor, Action! show, Indiana Jones show, Rock n' Roller Coaster, Tower of Terror, Fantasmic
Animal Kingdom...Expedition Everest, Kali River Rapids, Dinosaur, Primevil Whirl, Kilimanjaro Safaris, The Lion King Show, It's Tought to be a Bug
Epcot...everything!
Magic Kingdom...everything!
I would give them a book and let them read about each thing and let them choose what the must dos are and then if there is time for more, great.
I would skip the water parks as well....
Good Luck!
