Vaccination and Immunisation
Oral polio vaccines are delivered in the site of the potential infection. You may think about them as about a realistic simulation of the infection. As it is not scientifically sound explanation, please see the answer about whether intramuscular vaccination leads to intestinal IgA secretion which protects against intestinal route of infection.
Oral polio vaccine (Sabina) contains attenuated (alive but weakened) polio virus, which can proliferate and compete in the environment with the really dangerous wild polio virus. That way also people which has not been  directly vaccinated may be protected - both by development of immunity after less severe infection, and by reduction of exposure to the wild virus.
The route of delivery is much more acceptable by children.