Boys voluntarily tidying?

Just a short one today :-)

I was doing a late shift and there were three of four kids left in the library when another three walked in, two boys and a girl. They sat at the desk opposite from where I sit. They weren't being too noisy but there were some strange sounds coming from there so I looked over my monitor like a timid giraffe peaking over a fence.

One of the boys spotted me looking at him and changed tack from whatever he was doing.

Boy 1 says to Boy 2 "Boy 2, I dare you to tidy up the library!.
Interesting I think. I've not come across this before so I say, "What does he get if he does it?"
"A small bit of his dignity back" he replies with a snigger.
At this point I would never think in a million years that this is going to woek and then:
"OK" Boy 2 answers and promptly starts tidying up the library. Boy 1 looks me straight in the eye (that description is a bit of an exaggeration 'cos I can't see that far, but he was definitely looking in my direction) and says to me, "You owe me."

Now what should I have said to that?

P.S. Boy 2 spends about 15 minutes tidying up the library until I am actually impressed, sits down next to boy 1 who says in a deadpan voice, "You've actually lost dignity now".

Go figure.

Librarians 5, Children 3