Growing up and having dogs my whole life the rules always were that if our dogs bit unprovoked they would be put down. Little harsh I know but honestly if one of the dogs ever did attack a child I don't know what I would do. Just knowing those were the rules growing up my parents and now myself work very hard at preventing these behaviors. High prey drive is no excuse for biting, Meiko has a child nipping incident on him from when he was younger but this is why we spent countless hours and a lot of money with behaviourists with him.
I have met Harley in person and he is a spectacular dog but judging by your concern I think it's time that he gets some major behaviour modification done, it's never too late.
Prey drive can be harnessed into something good. We were and are lucky enough to have Meiko working cattle occasionally and this has helped a great deal, my biggest dog motto is "tired dogs are good dogs"
I am currently concerned about Karmas temperament and guardian nature right now and have found by changing the "rules" and buckling down with her has made some improvements, she can not be trusted offlead at all at this current time so tiring her out is quite difficult as walking does nothing to both physically and mentally drain her...
Time to get tough and creative with him IMO
Prey drive or not he needs to know the limits
Courage is being scared to death and saddling up anyways. ~John Wayne