I just hope the "flak" didn't happen in the child's presence. Perhaps you should ask the ones who are complaining just how they think the situation should have been handled, but you'll have to ask it in a way that expresses curiosity rather than an accusation, or they'll just light into you again. I think a lot of times issues between adults about how a child should be disciplined are just a power struggle between the adults. They resent that you exercised this authority, even if they didn't have the gumption to do anything about the behavior themselves. Kids will pick up on this and the bad behavior will escalate if it seems that in doing so he can manipulate adults into a blow-up.