Surely even secular children have heard of Santa Claus.
I tell them that Jesus is just like Santa Claus, in that He sees you when you're sleeping and He knows when you're awake, and always watching everything you do. But He is better than Santa, because if you're "good" and obey all His commands, instead of presents you get to spend all of eternity praising Him for being perfect! And if you don't, then instead of coal in your stocking, you get to be burned alive for all time in His Lake of Fire!
This discussion is usually followed up about a week later when the parent brings the child to me and tells me that she's having nightmares about being on fire every night and begs me to fix the problem. (Or, more often from atheists, threatens me if I don't.) I take the child aside and help her become a True Christian™. No more nightmares about Hell!
Usually within a month, the child has convinced her parents to bring her to church and stay through the services, because the child is terrified Mommy and Daddy will go to Hell.