Oh dude, you would not believe the trouble this topic brought up last time.
Anyway, depending on the size of the kid, many times, I let them RIDE IN FRONT.
There, I said it.
When mine were big enough to ride on the back, I either had them hold onto my belt or at one time I had a nice belt with handles built in, "Luv Handles", and they were so nice, they were stolen pretty quickly.
Then, there are these that clip on your existing belt -
Luv Handles Motorcycle Safety Accessory Homepage
Easy eough to make your own -
Still, if the kid is small enough, they really like riding on the front and I felt safer and more in control of them that way.
There is more, but man, last time, it got a little out of hand around here....