There are unfortunately many stories of creeps trying to lure kids into their cars or something similar. And it usually starts with a smile & a "hello".
I believe you're wrong. The "creep in a car" is a caricature of abusers that has developed over the years, when in actuality an abuser is likely a relative or teacher of the child, someone who can develop a relationship and gain trust.
I'm not suggesting you encourage your child to chat with strangers, I'm saying telling them not to respond to a good shabbos reeks of misplaced paranoia.