Could you tell me if vaccines work, how comees an unvaccinated child pose a risk to a vaccinated one?

Partial protection: Some vaccines give partial protection to some kids and these will be susceptable to infection by wild disease carriers. Some of the measles this year in houston were traced to an imported case infecting kids with no, one or two doses. The chickenpox vaccine has essentially ended CP deaths (were 50/yr) but we still see breakthru cases, all of which can develop secondary illnesses like invasive strep.

Immunity: Many very young children are not yet fully immunized. They need the full series of vaccines over the course of a few years. In addition, some vaccines provide herd immunity where other people who are immunized provide some protection to younger people who are not yet completely immunized.