Children of any age can go on the Kilimanjaro Safari ride at the Animal Kingdom. There is no height requirement for this attraction.
For your general knowledge, you should know that you can take your toddlers with you on any ride for which there is no height requirement.
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/size 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.
Have a great vacation with your family!