I would be utterly devastated and furious if someone came up to hit my child, despite what he did to another child - there are times I don't even SCOLD a stranger's child at parks/playcentres etc, due to the possible repercussions from their parents in our "blame everyone/sue everyone" society. Never mind the ones who could use a good smack (and I'm anti-smacking LOL).

But can I just jump in here to say, as someone who works with children (and their parents are more an issue believe me!), that you did the right thing at the time, such as trying to find a police officer - but other than monitoring the park situation to see if that woman reappears, I personally wouldn't take it further - and your son has probably already forgotten the incident and moved on, so I would leave it alone for his sake actually.

But privately, call that woman a lot of horrible names, if it makes you feel good LOL.