I do not understand the 'cannot reach in the back of my truck' line.
But still, talk to children. Tell them any time an adult is asking a child for "help" (to find a puppy or whatever), do not go near strangers approaching you. It is not real. They do not need help for anything. Something weird is going on.
I am a grown adult veteran and whenever a stranger stops and starts approaching me or talking to me, even as an adult I am very very careful about that. Children should be ten times more on guard.