Hi Marleah!
I suggest visiting this Walt Disney World web
page. It will give you tips on traveling to Walt Disney World with very young guests. Be sure to click on the "Attraction Height Requirements" document on the right side of the web page. That will show you the height requirement for those attractions at Walt Disney World that have one; as you can see most do not. Thus you will be able to do most of the attractions with both of your children! I realize that may sound strange since this is your first visit to Walt Disney World, but it's true; most of the attractions were designed with the whole family in mind.
The only thing I would add is be careful with 3-D movies. The 3-D movies, such as It's Tough to Be a Bug in Animal Kingdom, always scared my children. They didn't like putting on the glasses and they didn't like the movie! The only exception is Mickey's PhilharMagic in the Magic Kingdom; it's fabulous and not scary at all. Also, the Haunted Mansion frightens a lot of young guests. It's not really scary inside, but I can understand why children think it is; my son was frighten just by the name of the attraction for many years.
A few more pointers based on my experience: slow down and enjoy
the experience of visiting with young children instead of running from
attraction to attraction; take your own stroller because your children
will be familiar with it; consider staying at a monorail resort or
renting a car to allow you to return to your hotel quickly to take a
nap, rest, etc; consider a character meal, but don't rush character
interactions until you see how your children respond; do those things
that your children find most interesting. The most important thing of
all is to just have fun!