What do you do when someone (a lot bigger than you) curses in front of your child?

You should have got up and changed seats. You can't protect your child from the world of bad words. although this person was out of line, you should not feel bad about not saying something. If it bugs you remove yourself from the situation or let I theatre employee deal with it. If you have talked to your child, he knows he is not to say these words but he will hear them.