It isn't just young children at risk. Some fraction of the people who were immunized are still at risk of catching the disease.

Getting an immunization doesn't guarantee immunity. There is a fundamental trade-off at play: make the immunization shot too strong and more people get harmed; make it too weak and fewer people actually get immunity.

The funny thing about the anti-vax crowd is they have opted to turn the dial to the maximum, and most dangerous, setting: use a full strength virus in order to gain future immunity.