Question about vaccines.

Sara • Mommy to Ayden Alexander & expecting a princess in Dec. 2015!
Just by curiousity, what problems or issues have you run into in your older children not having their vaccines? I live in CA, not sure if it's a requirement by CA or just the school districts I live near but before preschool and any day care enrollment it's required for the child to have all of their vaccines current before attendance. 
What has been your answer to this if its something you have come across already?