I teach sewing & quilting to 9 year olds to adult each week in an after school program. We allow even the 9 year olds to use the rotary cutters after instruction and with supervision.
They are usually more careful than the adults. Stress the importance of retracting the blade every time before laying the cutter down. We usually have to help them hold down the ruler while they cut. This allows "supervision" in a non intrusive way. Good Luck.