I think those sorts of laws would be state based, since it is the states that administer the schools. In which case it will depend on which state you anticipate your children going to school.

Perhaps check on the website of your relevant department of health, or failing that department of education.

Perhaps too try emailing a doctors surgery or school admissions office to ask directly.

Speaking (as I usually do) from the point of total ignorance, I would have thought that vaccinations were not legally required as patients have a right to refuse care?? Like you say, the immunisation is optional, but may be required for schooling...