I'm going to disagree. My wife have three off the track Thoroughbreds and one Oldenburg with which we event. After having been in the track environment and on race courses they seem to me to have much more confidence and independence than the warmbloods. They become "bomb proof" much more quickly than the horse that spends its life in a stall and indoor arena. We vacationed at the White Stallion Ranch in Arizona when the mount they gave are kid was an off the track Thoroughbred, it did not surprise us much.
I’m glad you had a positive experience with your OTTB, but the original poster was suggesting that they might make kids’ horses. There aren’t a lot of kids who can take a horse from the track and reschool it, much less make it into a quiet mount.
I too have OTTBs and love them. More and more middle-aged riders are giving them up in favor of draft crosses, WB crosses, Cleveland crosses, Appendix QHs, etc. because they get tired of the “Thoroughbred moments”—the spooks, shying, and silliness some Thoroughbreds are prone to. The ground gets harder when you’re over 50, and orthopedic surgery is painful and expensive. Sometimes the fact that the TB is cheap is outweighed by the fact that they are higher strung and harder keepers than, say, a QH.