My child tried to lie on top of me on the mat and I’m worried he may have had some heat transfer. He’s five. Is that a concern?

The FIR Rays cannot penetrate through your body and do any harm to your child. 

They can bring benefits, but it is not recommended to use any heating procedure with children without a doctor’s approval and your presence.

The main rule with children is to never have the mat on heating mode without supervision. A child can lay on it and overheat.