kids and adult tredmill

I'd be concerned about getting off the treadmill, actually, as it can be a little dizzying to get from the moving belt to the stationery ground, particularly if you fail to do a cool down phase (which I can see a kid skipping).

Plus the child, I'm sure, should be tall enough to be able to comfortably grip the sides of the device in order to keep from falling.

As with any exercise regimen, I recommend speaking with a doctor (in this case, the child's pediatrician) first, as other exercises, such as signing up for soccer or hockey, might be preferred. But a taller, older child might be okay. Again, though, ask the doctor first. The child would also have to be responsible to some degree -- you don't want him or her jumping up and down on the treadmill or using it as a launching pad or otherwise using it unsafely or potentially damaging the equipment.