Unfortunately, that's not how it works. If only unvaccinated kids got the diseases, then we could happily let Darwin deal with the situation. But vaccines are

Unfortunately, that's not how it works. If only unvaccinated kids got the diseases, then we could happily let Darwin deal with the situation. But vaccines are not 100%, and the amount of immunity varies from child to child. The systems works when the percentage of immune children is high enough that the chain of infection is broken before those for whom the vaccine hasn't been effective are exposed (see: Herd Immunity). Anti-vaxers break down this protection, and infect not only the other unvaccinated, but also the vaccinated who don't have full immunity.