That's what I thought!

"A bad thing happened while you were out riding your bikes," I told my kids.  "The cops drove past and saw that all three of you were out on your bikes, scooters, and skateboards without your helmets.  They tracked me down and tried to haul me off to jail.  I told them that you would all wear your helmets from now on if they let me stay and be your Mom."

The kids were all terrified, and have worn their helmets every day since without fail.  Now I just need to find a way to get them to wear their shoes...