Pirates fireworks cruise.
Do the 3p parade from Frontierland...buy a turkey leg about 20 minutes before the parade and split it among the 3 of you as you wait in one of the rockers. Best place to watch it from (I think).
The Luau may be fun for you guys.
With respect to the water parks, Blizzard Beach has a better "Toddler Zone" in my opinion...though I think Typhoon Lagoon is the better park.
At EPCOT World Showcase, not only can you take time to find all the "hidden" meet and greets, but you can play the Perry game. Only thing is...you'll only get one phone. So, be ready to have them "share" it.
At EPCOT communicores...do the piggy bank. Also, stop by the cool spot and "try" a beverly!
If they are anything like mine, they'll love the water play areas. So, be prepared for that. There are several at EPCOT Futureworld, and a large one at Magic Kingdom (Casey Jr's). Put them in clothes that dry easily, and you may even want to pack a small towel (I carry a golf towel generally)...
When you get on rides, you can ask the CM to be seated in the front. However, at those ages, you are going to run into height issues. Pre-check ride heights so you can temper expectations. Also, Child Swap may be something you'll want to become familiar with as the 4 year old may be able to / want to do things the 3 year old can't yet.
Also, they do offer in room babysitting and child care centers. So, that's an option to have a few hours of "alone time" with each child doing something they are more interested in...or if you need some "alone time" for yourself!
Animal Kingdom is going to require a lot of walking, and in the summer it's very hot. You may want to consider a stroller for that park.
Which resort are you staying at?