It's easy to assume that people who don't have their kids vaccinated are too stupid to understand risk properly.
But even if you know the risk attached to vaccination is small, as soon as you have your kid vaccinated you become directly responsible for anything that goes wrong. Whereas if you don't have them vaccinated you can change your mind at any time - sins of commission feel worse than sins of omission.
And of course if everybody else has their child vaccinated, then the chance of your child catching anything is very small. From a coldly rational point of view, your least risky position is to be an unvaccinated person in a community where everyone else is vaccinated.