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 am likewise a teenager(16), and cannot agree more with what Kevin said. NEVER, EVER force your kids into doing an activity that they don't enjoy at all. If your young child wants to do ballet instead of little league(or vise versa), let them! Aikido is especially nice because its not traditionally or practically gender specific.
This said, I think that children can be very self motivated to do martial arts.I don't have any experience of late with other children taking martial arts, but ten I was six, I was pretty interested in them, possibly due to the Ninja Turtles, or some other influence that would get to a six year old. Well, my mom found that at the YMCA, they were teaching some sort of karate(no idea which style...it was japanese, though, and stricking, not grappling). I took it for maybe six months, earning one belt, and becoming rather good before I moved. I don't remember, and neither does my mother, any time when I lost interest in it, or requested to quit. In fact, when we got to our new house several states away, I started taking a sort of tae kwon do at a local dojo chain. At no time was my mom telling me, "YOU HAVE TO DO THIS OR ELSE", or even implying it. It was entirely self motivated, and I think that had my mom tried to get me to play baseball, I would have hated it and quit almost immediately, whereas she couldn't tear me away from martial arts.
So, in my opinion, you should let children be trained, though having a good sensei really helps. Don't try to train them if they'd rather do something else...I've said that about seven times in this message, and its still just as important. And also, when kids are happy, they can be a lot less figety than you'd expect. My other suggestion would be, as has been noted elsewhere in this thread, to play high energy games with the students at the end(preferably) of class as a sort of reward. When I did TKD, the instructor used to let us do a lot of flying kicks before we went home.