After the absolute basics (where the brakes, clutch, throttle and gear stick are) clutch control is key. I recommend for the first 5 minutes or so, and you can do this for as long and as often as they need, bar them from using the throttle. Clutch only. And explain what is happening, that they are engaging and disengaging the engine from the back wheel and that it is not simply "on" or "off", there are grey areas of great value. Throttle and brakes are easy, they are "go" and "stop". The hardest thing to understand is the clutch.
Something else fun you can do is sit on the seat behind them and get their feet and hands where they are supposed to be, and you can show them what its like by controlling the throttle/clutch/front brake with your hands. Then you sit behind them and let them do the controlling, then you let them go completely.
Kids are quick learners, as long as its fun theyll get it.