while I certainly have theories about how this developed, they really don't matter.
IMO, you should stop using the -no bark collar and find a trainer. A dog being aggressive towards children is best handled in person.
For now, manage the behavior by keeping the dog away from kids outside the home. It is your responsibility to keep everyone safe. This could end up bad for the kid, the dog and you.
When a dog saves the life of a man, it becomes clear that partnership knows no bounds.
Fama - T.E.D.D. OEF XI-XII (GSD)
Marshall - T.E.D.D. OEF XII-XIII (Lab)
Lucian - Med Alert (Cane Corso)
Pud - the old man (Pit x Lab)