I agree that you may not be able to teach a 2, 3, 4 or even 5 year old anything in a lesson. However, the important thing is the exposure! My parents had me on a horse at any chance, even if it was my mom's 17 hand dressage horse. At that age, just learning how to "hang on" without hanging on the horse's mouth is great. Take it at whatever pace the child needs. Sounds like chickenranchwife and I were on the same page... I was barley five when I did my first off line show and I showed with 10 year olds (and got second!).
The more exposure the better... she will have better horsemanship and riding a horse will be as natural as walking on her own two feet. Even better, she will have "horse sense", which will help keep her out of trouble throughout her riding days.