How much should I give the 'accidental' model if I win the contest?

Question:

How much should I give the 'accidental' model if I win the contest?

I was at a beach party and saw this young father with his baby sleeping on his chest. Beautiful moment. I took some photos and am entering one of them in a contest. If I win any money, how much should I give this young man? Percentage?
Appreciate advise from those on both sides of the camera.
Thanks

Best Answer - Chosen by Asker: If you know these people you may want to have them sign a release form giving you permission to enter the photo. (Just to be on the safe side. As to whether you want to give them any compensation from winnings is up to you.