I feel your pain. those people make me so angry too.
I think it is OK for your kids to see you in this situation though (hard as it may be on all of them) There will come a day when some rude kid wil say something directly to them and they will handle it with either frace, diginity and self assurance or they will feel shame and humiliation over thier way of life. The way they react will have a whole lot to do with how they saw you handle it.
So hold you head high and simply tell the jerk that you don't wish for your child to learn the same bad manners as he/she learned in school. (OK perhaps you should save that one for when your children aren't around
hugs to you.
Don't worry about ranting. We obviously care or we wouldn't have read it. We all need a good safe place to rant about all the evil people of the world.