In reply to AAG353:
Let them hold the rope.
Although individuals vary... an adult tends to have centre of gravity around their belly button, which tends to be a few inches below the harnesses belay loop. So sitting on a rope in a sit-harness (abseiling, being lowered) is usually fairly secure.
Kids, and more so the younger they are, tend to have a slightly higher centre of gravity which might be the same or a little above the belay loop of a harness. To stop the feeling of turning upside down they may want to hold the rope. Why not let them?
Also be sure that the waist belt is securely above hips and for the youngest kids an appropriate harness (which is likely a full body) .
Post edited at 16:13