I charge $5/half hour over 9 hours or after my closing time of 5pm, so I'd charge $10 in your case for an extra hour. However, if the little boys can play together and you can get on with preparing dinner for your family and whatever else you need to do $5 would be fair. I wouldn't give him another snack though, do you usually have one at around 3pm? Don't feed him after that and use the excuse that you might ruin his dinner! His Mom can feed him when they get home.

Especially don't let this become a habit because one thing we learn in this business is that once you let somebody have special treatment you are setting yourself up for it to happen again and again. We have to be fair to our clients but our contract policies and hours are in place because we are running a business, keep that in the back of your mind.