The issue with these mandatory vaccination rules is that they establish with them an exception to the principle that parents should be the ones to make the health-care decisions for their own children, or that individuals should be able to make health-care decisions for themselves. And when we make the move to allow for such exceptions, it's a slippery-slope that can bring with it a myriad of other anti-liberty mandates in the future.
While I agree with vaccinations, mandatory and enforced anything, especially health choices, are as you say a slippery slope.