We carpool with a friend of mine. She does AM and I do PM. My question is, if my son is sick, should I still be doing the PM pickups?
I've done several different carpools. In general, if your child is sick, you don't participate because a) you are home with your child, b) the rest of the carpool doesn't want your germs! There have been times when I've continued my part because the other parent/parents just couldn't make other arrangements or leave work early (obviously, when my kids were also older and it wasn't a big deal to leave them at home for 20 minutes.) Usually though, the parent of a sick child gives as much notice as they can and doesn't participate.
I'd sort out expectations with the other parent and try to have some fall-back arrangements in place ahead of time. It's inevitable that one person will have to miss carpool due to illness or other emergencies. After all, it's also possible that one of the parent drivers could be sick and unable to drive one day. If there is another parent, perhaps s/he could fill in.
I hate inconveniencing people if I've made a commitment, so as long as my sick child was cared for, I'd probably still do the carpool. Either I'd find someone to watch him/her for that time or, if s/he was old enough and not too sick to be left alone, I'd leave him/her. If I couldn't do the carpool, though, I wouldn't beat myself up about it. I would give as much notice as I could, apologize profusely, and hope for understanding. It's likely that the other parent will be in the same situation at some point during the school year. OTOH, if the other parent was home with a sick child, I wouldn't expect her to carpool that day. I'd understand that when you have a sick child, carpooling is not something you should worry about.