S.M.
I would be very tempted to handle it like this:
Child A throws a wood chip at your son on the playground. You see your son pick up a wood chip to throw back. You say "Honey, we don't throw wood chips" and then in a pointedly louder voice "I know A threw one at you, darling, but we don't throw them back"
Child B takes the toy truck from your son. Your son grabs it back and B starts crying. "Sweetheart, can you use your words to tell B that you will let him play as soon as you FINISH YOUR TURN."
You're teaching your son how to handle the other kids and at the same time making sure the other moms know the whole situation. And you're talking to your son instead of having to talk directly to the other moms, so you won't be as nervous.