Not sure they do it for Christmas, but this summer we went and paid £10pp extra to get in th park a hour early. We got to meet peppa and George, they played with the children in the soft play and playground (my daughter loved that peppa pushed her on the swing) and they got to go on the rides early too. By normal opening time it was much busier, so we were glad to have the quieter time beforehand.
I'd go off peak next time though, during school holidays is crazy although for some I know it's unavoidable.