vaccinations okay for 3 and 4 year olds????

hello-
i was wondering if it is okay to vaccinate pre-schoolers? both of my children received vaccinations w/out problems (thank goodness) as babies. now it's time for their check ups and i was wondering if vaccinations are okay for them since they aren't babies any more?
i told the pediatrician i wasn't too keen on their being vaccinated and he was not in agreement w/me. he's a very good doctor and i will not change doctors over this.
i would appreciate any insight on the subject.
thanks,
jlbrownfield