A comprehensive list of height restrictions for the rides at WDW can be found
here. There's a blue box on the right hand side of this page that says "Height Requirements." From here, you can download a handy pdf that lists everything in one place.
In addition to getting the height guidelines for the rides, I suggest that you invest in a good guidebook such as the
Unofficial Guide to Walt Disney World. When trying to decide whether a child should ride a particular attraction, there are factors to consider other than just height. For example, there is no height restriction on the Snow White ride, but many preschoolers are terrified of this. The
Unofficial Guide will discuss situations such as this. By getting fully informed, you can help your family have the best trip possible.