Hi - being the Mum of 2 "energetic" boys AND a childminder, what you have posted is very familiar territory.
When Bramble has had enough, or is taken by complete surprise, she will growl and nip - like all dogs.
I take her to school twice a day, and always am fearful of the odd child who will creep up behind......
I also know what parents are like - many would understand why it happened and just shrug it off - the child learning a valuable lesson. Other parents would make so much of an issue out of it "your dog bit my child and therefore is dangerous...."
You are taking action to avoid the situation re-ocurring. That is the action of a very responsible person.
I totally empathise with you here - if you need any more help and support - I know there are many here too glad to help.........