Unfortunately it happens , here is what I instruct my parents to do if that happens. I actually go over this in the new team meeting, in hopes that it will get the message across to the parents to never do it and give them the tools to deal with it when it happens.
Find out who the child is that they are there for. (easy enough). Start cheering for her, genuinely , strike up a conversation with them. Be super nice and supportive for the child they are there for, congratulate them etc... Again this all has to be genuine. Within a few minutes they will figure out who your kid is and feel stupid beyond belief, and be forced to cheer for your child as well (putting on a fake face). Do not ever let them know that you heard them. The feeling of stupidity will haunt them the whole way home and hopefully have a lasting effect. And you will feel good about being the bigger person, cheering for their kid, (it's not the kids fault the parents are jerks) and having the satisfaction that they will remember how stupid they are for a long time. Hope that helps.
Or you can just take a picture of them and put them on CB.