If the relationship between the child (usually son) and dad is slightly distant/cold, or they don't see each other much (i.e. father going off to check on his different estates, son in the care of a nurse etc) and the family is upper class, then I would go for Pater. However, I read a children's book published in 1949 and the boys (8-16) refer to their father as 'Sir' - just putting it out there... Daughters tend to use 'father', or so it seems in the historical books I've read.
