My now 2yo daughter started riding with my wife and I at roughly 6mo. This fall we started to let her ride on a lead line, and she did 2 barrel patterns at local shows lead line. I work with several non horse people and you would think that we were taking our child out and beating her. She wears a helmet, and has a properly fitting saddle (I had never seen a 6" saddle before). Wears jeans and boots.
My wife and I both were riding before we were in pre school. So we don't see a problem with it, and we are very particular about which of our stock she is on.