Hi Tara!
Generally speaking, a guest's ability to participate in a Walt Disney World attraction is completely based on height, though there are a few attractions were it is hard for very tall or very large guests to get into the ride vehicle.
There is a page of travel tips for people with small children
here. On that page, you will see a button on the right hand side of the page that says "Height Requirements." Clicking on this link will bring up a PDF list of the attractions in all four theme parks and both water parks that have height requirements. If an attraction is not listed, it means that there is no height requirement and any size child can enjoy the ride.
My two sons are 5 years apart in age and we never had a major problem with that, though there are definitely a few attractions in each park that only the oldest son could do, even today. That said, I recommend visiting the Magic Kingdom for sure. After that, I will say at those ages, one of my son's loved Epcot while the other loved Disney's Hollywood Studios. However, Disney's Animal Kingdom is also another great choice for you to consider.
By the way, even though there are is no height restriction, I do recommend caution with the 3D movie "It's Tough to be a Bug" in Animal Kingdom; a lot of very young guests are completely frighten by that attraction!
Enjoy planning and write back if you have any more questions!