Years ago, as a street photographer in Chicago, people were much different, more open, more receptive, curious and seemingly more interested in meeting their neighbors and fellow urban residents. Now it's different. People are suspicious, distrustful, tending to stereotype news clips to the general population, and shy. We didn't have to snipe from a distance with long lenses. Years ago, we asked people for permission to photograph them because they seemed interesting subjects. We introduced ourselves. We told them we felt that what we were doing was important and that we would appreciate it if they would allow us (me) to let them become a part of my project. Most were flattered to stop for a minute and allow their image to be captured on film. I enjoyed talking to them during and afterwards. Finding out about them. Even after I took a shot without first asking consent, more often than not, I went up to them afterwards, told them what I was doing and even asked them to sign a release and offered to send them a print or two of their photograph. Imagine that. People were receptive to that.
Sadly, I think you were a victim of urban hysteria; of a mother who has serious social phobias and paranoia issues and in this case, unfounded but genuine fears. Even sadder is that the sins of the mother are visited upon the children, like the kid who gave you the finger while he was walking away. What sort of person will that kid be when/if he grows up? Look at how he deals with strangers? His mom or whomever she is, is teaching him how to be disrespectful and rude. Nice. Soon to be appearing in the next generation. It's a shame. Can you disarm that? I don't know. Is it worthwhile incorporating a disarming sense of mutual humanity into your photographic repertoire and paying it forward? Probably if you haven't done so already.
My suggestion to you is keep on doing what you're doing but try to be a little more receptive to talking to your subjects before and after you photograph them and as someone else said here earlier, "Blow it off". No harm no foul. Certainly not everyone is like these people you ran into.
Take it light ;>)