Was it really a misunderstanding though? Sounds like a fairly well thought out plan by the kids in my opinion, and perhaps there's a life lesson in there for them?

I remember a similar situation when I lived in the mid-west. I looked out of my window to see a complete stranger shovelling my snow (and I planned to get my strapping teen to do it). I had seen the shoveller knocking on neighbour's doors after shovelling so I half-suspected it was an after-the-fact payment situation.

Fortunately I was able to tell him "Thanks but my son's just about to do that."