I am aware of herd immunity, but the same statement applies. You are asking parents to inject their children with something they think is dangerous. They are going to prioritize their own child’s health over a small percentage of other children, especially as social trust decreases over time.
What if me, as a parent, decide that it is insane my child to wear clothes because their skin will get fucked up from fabric? OK for my kid to walk around the school naked? What if I think it is in the best interest of my boy to be tough and tell him it is OK to punch the lights out of anyone he wants in school, other kid, teacher, whatever? I am a parent after all and I only care about my child - your or other children in the school are not my concern, all this good?
One of the functions of government is to ensure the health of its citizens. Potential harms have to be weighed against individual liberty. Parental choices can affect the health of their and other children. The line has to go somewhere. Some parents don't even believe in feeding their children: https://www.bbc.co.uk/news/articles/c62v59621dro