Hello Jim,
Welcome to the Disney Parks Moms Panel and thanks for your question! There are some attractions at Magic Kingdom with minimum height requirements, but I think you'll find that there will be more things the little Mouseketeers in your group can do than they can't do.
Magic Kingdom - Minimum Height Requirements Big Thunder Mountain Railroad: 40 inches
Space Mountain: 44 inches
Splash Mountain: 40 inches
Stitch's Great Escape: 40 inches
The Barnstormer: 35 inches
Tomorrowland Speedway: 32 inches to ride or 54 inches to drive (and must be 12 months or older)
A great resource for finding height and other restrictions for attractions at all 4 theme parks and both water parks at Walt Disney World is the
Attractions page, so you might take a peek at that for additional information before you visit if you have questions. These restrictions will also be posted clearly outside the attractions if you have any questions when you're there.
Wishing you a most magical day at Magic Kingdom, Jim! Please write back if I can help answer any other questions.
Beth