Agree completely with Gail...the horses are as "bomb proof" as you'll find anywhere (that means that, as far as horses are concerned, it's pretty safe).
For the experienced horseperson, this will likely be a boring experience. The group walks the entire way, the terrain is not challenging, and the horses practically steer themselves.
We have horses but still decided to try Disney's version of a trail ride. We took an early morning trip and did see some deer and a jackrabbit. The ride was fun because our expectations of it were not the same as what we'd have back home on our own horses.
Your 15 and 12 year olds should just love it. If you haven't ridden a horse recently, you will probably be a little saddle sore but nothing that will slow you down (IMO).