I don't think 6 is too young...I was definitely riding on my own (wtc) by that time, and I started lessons around that age too, even though I already knew how to ride.
As has been mentioned, you have to keep this group 'interested' in what is going on; make learning a new concept fun, and even though everything has to be repeated in order to get it across, try to keep your cool, and figure out creative ways to reward them for good efforts.
Maybe set some guidelines for them, especially in the area of taking direction; it is for their safety and benefit that they listen to instruction, especially around horses. Sit down and talk to them with their parents present, and set forth the rules, and stick to them. For example if you have to ask a child to pay attention more than 3 times, then he\she needs to dismount, and sit in a designated 'time out' area...if they really want to ride, they will learn how to pay attention really quickly, because they won't want to have to get off!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."