The trail ride that goes from Fort Wilderness is something my elder DD did several years ago, she did enjoy it, but she was a fairly experienced rider and said that she thought the horse were just too easy to ride. They all knew the route, what was expected of them, they never got out of walk speed and apart from the occassional go at nibbling on a tempting looking bush required almost no driving. She thought it was a pleasant enough ride in the countryside, but she felt like she'd done the ride on one of the horses from the carousel. There were a number of other activities for younger kids ( the trail ride has a minimum age I think of 9) at the "farm" where the ride starts and finishes. We entertained our younger DD by taking her into the petting zoo ( goats and chickens) lookign around at the other horses and they had some small ponies that the young ones could ride on laps of the small paddock they had, while you walked next to them if you wished.
If you want a more genuine "horse riding" experience I believe Grand Cypress Equestrian Centre (407 239 1938) or Horse World Riding Stables in Poinciana (407 847 4343) are both a more genuine experience. HWRS looks as if it is considerably nearer than GCEC, it also seems to be considerably cheaper, although I'm told GCEC is an absolutely first class establishment.