I find the following quote, from the article on vaccines linked to by Jonathan, below, horrifying:

In the wake of last month's outbreak, Linda Palmer considered sending her son to a measles party to contract the virus. Several years ago, the boy, now 12, contracted chicken pox when Ms. Palmer had him attend a gathering of children with that virus. "It is a very common thing in the natural-health oriented world," Ms. Palmer said of the parties.

I had chicken pox as a kid, and I remember it as a very unpleasant experience, to say the least, and I didn't have an especially severe outbreak. Measles, I take it, is worse and also more dangerous. Parents like Ms. Palmer are not only exposing their own children to horrible illnesses easily preventable by vaccines, but they are putting other children, including my own daughter, at risk, since the measles vaccine is only 95% effective. (And what about adults who either received an ineffective vaccine or immigrated from a country where vaccination was not universal)? A Ms. Carlson says, "I cannot deny that my child can put someone else at risk."

Without the externality of putting other people at risk, I think mandatory vaccination would be a close call. With it, I'd say that unless a parent is going to keep his children at home and not expose them to vaccinated children, make them get vaccines. Too unlibertarian for you? Make them pay a fine equal to the monetary value of the level of risk to others they're creating, to be used perhaps to subsidize vaccination programs for the poor, thus reducing the risks from elsewhere.