They sure can.  

To order a PIVOT Tennis kit, you must be 13 years old.  

But to use a PIVOT Tennis Kit, you'll need to fit into the chest harness, armband, and glove.  The Small version of the Kit will fit a child who typically wears children's clothing sized 14-16.  If your child is much smaller than that, chances are good that the sensors will fit too loosely and not track movements accurately.