Why does my child need to be vaccinated against disease no one gets anymore?

Theyr'e not. I think you are referring to polio. While polio does not occur in the U.S. Anymore, it still infects other parts of the world. Infection can spread once again. Once a disease is totally eradicated from the world, vaccinations are not necessary, i.e. Smallpox. The present vaccination recommendations prevent still present diseases in this world.
B/c they DO get them. Whooping cough, meningitis, measles, mumpes, and rubella are all diseases that still occur. Children still die from these diseases or are left with long term neurologic disabilities. Most of these episodes could be prevented with vaccination. It is a big misconception that these diseases do not exist anymore, and it is a potentially fatal misconception if it leads to failure to vaccinate.
It's a small world. Diseases that used to be prevalent have been reduced by herd immunity. This resistance to a disease is the result of a high percentage of the population being immune, making it difficult for the disease to spread. Once the vaccination rate goes down the population is again susceptible to imported diseases. The only disease that has been eradicated is smallpox.