If this only affected their kids, I'd actually side with them and say vaccines should be a parent's choice. I'd still call not vaccinating the wrong choice, but it would be their choice. However, a parent who doesn't vaccinate their child weakens herd immunity and exposes other people to diseases. People who a) are too young to be vaccinated, b) can't be vaccinated for valid medical reasons, or c) were vaccinated but the vaccine didn't "take" (vaccines aren't 100% but herd immunity usually covers the fraction who don't get protected). When their actions negatively affect other people - including possibly killing them - they don't get to argue that it's their freedom to choose not to vaccinate and it goes beyond a "Darwin effect."