Failing to vaccinate a kid (unless there are legit medical reasons) should be a chargeable offence in the same way that letting them sit in the backseat of a car without a booster / seatbelt is. These parents, as stupid and credulous as they are, have endangered their kids and some of them might suffer life altering injuries or death from that.
"Here you go Timmy. Just shoot any child that gets close to you. We are scared enough of measles to give you a gun, but not scared enough to have you vaccinated."