You are SO much better than me. The first time I would have let the mother handle it, maybe even the second time without speaking up. But I don't care after that point. It someone else's child lays hands on my kid and the parent doesn't immediately remove their child I will ALWAYS speak up. I won't discipline the other child by saying things like, "it's not nice to do that or please share". I would simple say, DON'T touch my son in a very firm voice. I don't even care, flame me all you want I would never stand for that! Ever!
"Courage is not the absence of fear, but rather the judgement that something else is more important than fear." -- Ambrose Redmoon