So unvaccinated kids infect your vaccinated kids with diseases that they are supposed to be vaccinated?
The short answer: Yes.
The long answer involves two factors:
First, unvaccinated, disease-ridden older children who go near children too young to be vaccinated (or children who could not be vaccinated for health reasons such as a compromised immune system) cause those children too young to be vaccinated to become infected, and often to then die horrible painful deaths. This is the fault of the parents who refused to vaccinate their child, and said parents should be arrested and put on trial (and then imprisoned) for said death in the same way they would if they'd been planting random explosives around the suburbs and had blown the children up instead. The net result is the same, even if the explosions are slightly less painful.
Second, when a sufficient number of unvaccinated people become infected with a disease, this weird thing called "mutation" happens, and the vaccines become ineffective as the new strain created by irresponsible parents refusing to vaccinate their children then spreads unchecked. This is why the Flu vaccines that are available need to be re-done every single year, and while it's entirely possible for it to happen with even a single infected carrier, as more people become infected, it becomes more likely. Again, the parents responsible for this should be put on trial and imprisoned in exactly the same way they would had they used their own personal lab to cook up a bio-weapon and spread it around town to slaughter the neighbor's children.
Yes, it's true, you're totally free to go and build your own explosive devices*, or cook up designer virii** in the privacy of your own mad science lab, but it becomes illegal once you start spreading it around town where people will get hurt. The same thing should apply to deliberately allowing your children to go unvaccinated - if you are turning your children into a public health risk, you should be keeping them away from the public that you're risking.
* Technically this does require a license to work with explosives in some areas, but not all.
** This one however, there are currently no actual laws against that I am aware of. Yet. Most laws like that show up because someone, somewhere, demonstrated that it was necessary.