You will have so much fun with your little ones! My children are the same ages and it's a very magical time to visit 'the World'.
I'd recommend checking out the
Playhouse Disney show in Disney's Hollywood Studios. It's always a lot of fun for little ones, and it's just been completely redone. The show is all new and my daughters can't wait to check it out!
I also recommend taking the time to explore the Innoventions halls in Epcot. They are often overlooked and really hidden gems of the park. Your kids can explore and learn while they play, and the whole family will have fun together. If you visit the robots your children can even build their own as a souvenir! While at Epcot, also be sure to visit the Kidcot stations in each of the World Showcase countries. They will have opportunities to make crafts related to each of the guest countries. They will also be able to interact with natives of each country. My daughter picked a word that she wrote on a paper, and then asked a Cast Member in each station to write the same word in their own language. Educational fun at its best!
Lastly I would recommend The Festival of the Lion King in Animal Kingdom. It is a must-see at least once per trip show. My daughter was not yet 2 the first time she saw it, and she was captivated! We have more video footage of her dancing along than we do of the show itself! If you get a seat near the front it's even possible that your child could be selected to participate. My oldest has done this more than once and she is always delighted to be a part of a real Disney show.
Have a great vacation!