As far as I'm concerned the Dr. can run his practice as he pleases. But, how is an unvaccinated child a risk to one who is vaccinated? If the vaccine works, the vaccinated child is immune to the disease. Exposure so a measles carrying child should pose zero risk to the vaccinated child, ...if the vaccine works.
so = to
There are a couple of reasons for that.
For one thing, some of that doctor's patients might be infants who have not been vaccinated yet because they are being seen for the first time. To an infant, contact with any unvaccinated person is a risk. That's because it takes the immune system about two weeks to "kick in" following a vaccination. The parents of that infant have no way to know if other children in the waiting room at the doctor's office have received their vaccines; a non-vaccinated child can have a potentially deadly disease and be contagious even if asymptomatic.
The other thing is that vaccines are not 100% effective. This can be because of individual responses to vaccines (some people just don't respond as well as others), or because the full vaccination course was not administered. Some people cannot be vaccinated for medical reasons. So, for the child who is not protected despite vaccination or who cannot be vaccinated, exposure to an infected non-vaccinated child can be deadly.
From a public health stand-point, the goal is to vaccinate enough of the population to inhibit spread of disease. "Enough" depends on the disease itself, and can be from 80% to over 90%. As long as enough of the population has been vaccinated against a specific disease, the chance that a non-immune person will contract the disease and spread it to others is greatly decreased.