I kinda see that as child abuse.
What's especially insane about it being difficult to enforce is that you're hurting other people's children as well, even those who have been vaccinated.
Vaccines don't provide 100% immunity; oftentimes the realistic target is to merely achieve
good enough immunity across a population that cases won't tend to propagate into outbreaks. This situation where not all individuals are immune, but enough are immune to prevent outbreaks, is called "herd immunity."
Measles is insanely contagious: for herd immunity, we need 90-95% of the population to be immune.
Now, here's the problem: the MMR vaccine only achieves about 93% chance of immunity after one round, and 97% after two rounds. As such, MMR is on the very edge of being sufficient: it only takes a few non-MMR'd people to bring the immunity rate low enough to break herd immunity and allow outbreaks to occur.
When outbreaks have occurred recently, a large fraction of those affected are people who have been vaccinated, sometimes around half.
Anti-vaxxers aren't just putting their own families at risk, or even themselves and other anti-vaxxers. The movement is a threat to public health.