Although what you did wasn't technically "wrong", did you not realise what the potential outcome could be.
Firstly - they did nothing wrong in the eyes of the law. You did ! You threatened them - and most kids of that age will be smart enough to know they have you by the short and curlies once you say something like that.
Most importantly - you photographed underage kids without their parents permission in a park - and they could twist that any which way they like. I know because I have a teacher friend who basically nearly lost his job for a similar reason

If you publish that picture you may as well sign your own death warrant - no matter what the reason. I know if it was my daughter and someone had been taking pictures to post online (even if she was spouting every other word as an F-Word) - I would want to know who the hell thought it was right to take & publish their pictures without my consent.
You should have dealt with the situation differently... as in - just removing your kids from the situation ,explaining your reasons to the other kids - and being done with it.