Professional hat on here..
Dog, perfectly reasonable reaction, many well natured dogs would have bitten in that situation they are dogs, not robots.
However, a child is not ever able to be responsible for a dog, legally, you are, and whilst the child and her parent ARE morally responsible for what happened, unfortunately it could be argued that your son wasn't in control of the dog (as the dog was the other side of the bench to him where he could be approached without someone seeing).
I don't think thats right and it is unlikely, but if the childs mother kicks up a fuss, documents injury and makes a complaint, it is possible your dog could be subject to a control order.
Unfortunately the way local authorities and police forces deal with dog related incidents varies wildly from one county to the next - get sensible people dealing with such a complaint and it will rightly be tossed out on its ear.
Get dog hating idiots and the case could be very different.
If you were my client I would advise:
Get a trainer/behaviourist in to go through basic training etc, with dog and son.
Speak to local dog warden regarding any potential complaint and ask for advice on what if anything, you should do to mitigate such.
Write a statement of what happened in case it is needed later on down the line.
These steps will if ever required, prove you to be a responsible owner who acts immediately something occurs to take necessary steps to prevent it happening again.
In future, either don't leave your son in charge of the dog - not because he is at fault but because if anything happens, his account will hold far less weight than yours would, or ensure the dog is in front of him so he is able to protect the dog better.
This whole thing sucks quite honestly, I hope your poor dog is ok and your son too, some people are utterly stupid and take no responsibility for their actions whatsoever but prefer to pass the blame!