Hello, Kelly,
Congratulations on your 3rd child; we, too, visited WDW when our 3rd son was on the way! To help with your planning, I'd take a peek at the
Magical Beginnings Guide which highlights many family friendly attractions. However, the Magic Kingdom is a little outdated since it doesn't reflect the
new Fantasyland that replaced Toontown.
Also, you can search through the
Attractions at Walt Disney World if you'd like to explore them to open up other options for your older son in case he'd like to try a few other attractions. You will have the opportunity to change your searches based on thrill level, interests, audience, height, and accessibility/advisories (check rides for an expectant mother advisory).
By the way, I also really like using www.touringplans.com, which is an independent website that offers over 100 itineraries to help guide you through the attractions and the order to visit them to make the most of your days. This site would help you to make sure you don't miss any of the must-see attractions. I should note that there is a small fee for this service, but one that I've found worthwhile.
After looking them over, if you have any questions please let us know. We are more than happy to help!