Pardon me for jumping into the topic this late, but I just noticed this from a couple of days ago:
You may already have figured this out yourself by now: you could just as easily have reacted too quickly, when you still only thought something might be about to happen -- and now be feeling just as guilty that you didn't wait to make sure.

The way that I'm picturing it anyway, those kids had been doing stuff like that for a while now. A few seconds' delay in interrupting them this time wasn't going to do them much harm, while finding yourself unsure if you'd really seen anything or not would have put you very much on the spot. Without the details that you waited to observe, all you would have had to go on would've been your impression of what could have been happening and the controversy around the neighborhood would quite likely have been about whether you'd only imagined it.

It sounds to me as though you are (or were, anyway, when you posted that) beating yourself up for doing the right thing. Well (I figure) if you must beat yourself up for something -- then better it should be for having done the right thing!