Hi, Kristen! The Tri-Circle-D Ranch at Disney's Fort Wilderness campground has a variety of activities, and the pony rides are quite popular. Unfortunately, because of the petite nature of the ponies at the ranch, they do adhere pretty strongly to the under 48" requirement. With one of my daughters being a tall kid (she's already 5'8" at 12 years old!), I feel your pain.
You might want to consider the wagon ride. These evening rides are like an old fashioned horse drawn wagon, which takes you on a tour of the campground. They don't take reservations, but fill on a first come, first served basis.
There are also opportunities to tour the stables. There, you can see Cinderella's team of ponies, as well as some of the other horses that live and work around Walt Disney World.
More information can be found by clicking
Tri-Circle-D Ranch.
Have an awesome time!