Sorry to hear about this RL, it's never an easy situation when dealing with possible aggression in our beloved animals.
I agree with a lot of the information you've been given here as far as helping Harley deal with his anxiety towards children.
The only thing I want to add is that you need to be extremley cautious while taking Harley to parks, regardless of whether or not he is wearing a muzzle. If his trigger is children playing in the parks and laughing, screaming, running around; you will need to be extremley aware of his reactions and body language before you even reach the park - innterupting any unwanted negative behavior at the exact correct moment.
In your situation, if you don't know which signs to watch for, I highly reccomend getting a hold of a behaviorist that will be able to help you watch for those signs and help Harley understand his role around children.
As for what I would do personally if a dog bit a child? I agree with BenMax on this one. I can't say that I would be quick to put the dog down, however it would depend soley on the situation, the dog, and the severity of the bite and the dogs past history with this sort of thing.
"If you are a dog and your owner suggests that you wear a sweater. . . suggest that he wear a tail."
Vali (American Bulldog)
Artiro (Cane Corso)