Since I am only a teenager, I think the most important thing is that parents don't force children, or teens, to train. I am only 14 and I train 3 days a week and a least a min. of 2 hours a training day. My parents don't force me to train 2 hours a training day. I actually decided to do that. Don't force it. If children don't want to learn, don't force them.
I would also like to respond to Ron's reply about weapons work. I think that weapon work is great for kids. Don't think that children and weapon work are bad. Before a child can set his hands on a jo or bokken, there are rules he\her has to know. Usually things like these weapons can actually hurt someone, or if you goof around you and your partner will have to sit at the side and watch everyone else train. These rules are here so that no one will get hurt and usually everyone follows them. At one point in my training with the children classes, we would start with the bokken at warm up. The whole class would do different types of cuts and stuff. The dojo would also put weapons work on children tests. Stuff like cutting and position of the bokken. I even remember my 5th kyu test. I had to the "Kata" which was a 25 step jo kata. On my yonkyu test, I had to do the kumitachi 1-5, and possibly on my sankyu test I might have to do the Kumijo 1-6.