Without more facts it is hard to say whether or not the father was in the wrong. My kids rode a tractor at a very young age. It almost always was setting between my legs on the seat and me driving real slow around the barn yard or pasture.
I have seen some guys do things with kids on board that scarred the heck out of me. The ATVs are even worst. There is a guy right up the road that lets his little kids ride his 4 wheeler. They can't even reach the foot peddles but they ride them wide open. They many times are just hanging on by the handle bars.