A guest's ability to participate in a WDW ride is completely based on height, not age.
If you look at this page of travel tips for people with small children found
HERE,
you
will see a button on the right hand side of the page that says
"Height Requirements." Clicking on this link will bring you to handy
PDF that you can download. The PDF lists the attractions in all four
theme parks and both water parks that have height requirements. If an
attraction is not listed on this document it means that there is no
height requirement and any age child can enjoy the ride. The height
requirements are also listed on all the maps at the parks and on
signage in front of the various attractions.
As you can see on the download, the list of rides that are off-limits
for preschoolers is very small. However, many of the "bigger rides" are, for safety reasons, prohibited for little ones.
Some of the rides that you can all enjoy together include Buzz Lightyear, Toy Story Mania, Pirates of the Caribbean, Jungle Cruise, and Kilimanjaro Safari. These tend to appeal equally to all ages.
Have a great trip!