Hi April,
If you are only planning to visit one theme park while you are at Walt Disney World, I think that it has to be the
Magic Kingdom. This is the theme park that most people associate with Walt Disney World, complete with Cinderella Castle. There is so much to do at this theme park for all ages!
If your girls enjoy more thrilling rides, they will likely be happy with the Magic Kingdom mountains: Space Mountain, Splash Mountain, and Big Thunder Mountain Railroad. Also do not forget the Seven Dwarfs Mine Train, which is a new and more gentle roller coaster. If they do not enjoy thrilling rides, do not worry, there are many other attractions that appeal to older children as well, such as Pirates of the Caribbean and Haunted Mansion. For your young son, Fantasyland will probably be the place to be. There are many attractions in Fantasyland that appeal to the toddler and preschool set, including The Many Adventures of Winnie the Pooh, Peter Pan's Flight, and Voyage of the Little Mermaid.
Because you have such a wide range of ages, I do suggest that you split up for at least part of the day. That way, everyone can experience the attractions that might not appeal to the whole group. You can come back together for attractions that everyone might enjoy together, such as Jungle Cruise, it's a small world, and Dumbo The Flying Elephant. I also suggest that families come together for a meal, such as lunch or dinner. One of my favorite hidden gem restaurants in Magic Kingdom is
The Plaza Restaurant. It is quaint and features good options including great sandwiches, salads, and entrees.
I would love to help you plan further, April; however, I am limited in the space that I have to answer. Please do write back if you have any further questions!