He has never lunged at a child, but if given a chance I know he would run them down, what would happen next I do not know, nor do I ever want to find out.
Your gut instincts are usually the ones that lead us to the truth. You know Harley the best, and it really sounds like he has a fear and an insecurity around children.
And you're also right - a high drive doesn't make them aggressive, but when that is coupled with fear and insecurity it can easily trigger aggression in a dog. There isn't just *one* type of aggression.
Sometimes if a dog does not understand something, they want to make it stop...which is why they will become anxious at certain sounds or objects. (Flight reaction). If they can't get away, some will fight. Which is obviously bad when that frustration/fear is directed at people or other animals.
I agree that it's best to keep him away from things. You know your dog best, what he can/can't handle.