Quite some time ago I was asked, as the supervising rabbi of a Jewish early childhood education center, to approve the admission of a new preschooler whose parents refused to immunize him/her. In the public school, all you had to do was sign a waiver form that declared immunizations to be against your religious beliefs, and the school system had to let you in. But this was my synagogue's school, and the rabbi sets the religious standards. I required immunization, and we lost the family from the school.
Recently, as I took my own child for immunizations, I recalled this fact. My child made all the same arguments to me: "Now one has measles any more!" "Why do I have to get this shot? It hurts, and it does nothing!" Etc.
The simple truth of the matter is that the child who does not get immunized, and does not get the disease, can not claim that immunizations don't matter. They are benefitting from the fact that all of their classmates did get immunized, and therefore they are not being exposed to the pathogen in question. They are still very vulnerable and present a risk to the community. To bank on the fact that no one else has the disease is the height of irresponsibility as a member of any community.
Many myths circulate against immunizations, but the truth is that they are safe - and certainly safer than the old days of polio in the public swimming pools. The world population boom, and the decrease in childhood diseases are objective proof in part to the efficacy of immunization.
Judaism demands protection of life - all life - in the form of pikuach nefesh. Leaving one's child vulnerable to avoidable infection, and sending them out into public where they could then infect others would be an egregious sin if done unknowingly. To do it after being given the facts would be tantamount to intentional endangerment of a minor and reckless endangerment of the public.