Capture the Moment...but what about permission?

So yesterday was my first photography class. Seems like it might end up being a worthwhile class. First, digital cameras are allowed, so that means no need to confiscate my wife's Canon Rebel Ti for 8 weeks. Second, although we will obviously be getting a bit technical in the next class or two, we are going to focus mainly on technique and creativity. So the first class mainly consisted of introduction, and us going through a list of 15 "rules" (be ready, rule of thirds, get close, etc), along with some samples demonstrating the different rules. Mostly stuff I already know, but it was a good reminder because I often don't practice them (in the heat of the moment, it's hard to keep cool and think carefully about your shot).

So anyway, today I was at an event taking some pictures when I started thinking about the rules. One of the rules was "capture the moment", and that got me thinking. The moment wasn't about the event itself, but about the children watching the event. So with that in mind, I started shooting the kids...capturing the moment. When I got home and started looking at the pictures I found I had some great ones there, but I got to thinking...what about permission?

When is it necessary to get permission? These kids were in a public place, so certainly there is nothing illegal or unethical about taking their picture. However, what about posting them to my online photo gallery? I feel it is well within my rights to do so, but in an attempt to be sensitive to parents concerns about the dangers of the internet (sexual predators and all) what is the ethical thing to do? Ask permission (when I legally don't need it) and risk them saying no? Just go ahead without asking their permission?

I posed this question for discussion online here:
http://forums.dpreview.com/forums/read.asp?forum=1031&message=10581801