Having a vaccine is like wearing a seat belt in your car. The seatbelt doesn't prevent you from getting into accidents but it does minimize the harm caused by the accidents. In the same way, you children might come in contact with other children that have a disease, if they are vaccinated for that disease then the child won't be harmed by it.

There is also the issue of Herd Immunity. even if YOUR child never gets sick from a disease, the more children that are vaccinated, the safer everyone else is. Lets say that 1% of children CAN'T be vaccinated due to an allergy or other medical condition. If the other 99% are vaccinated, then that 1% is still safe.