It was probably stated on the menu or in a sign that a gratuity charge was added for groups over X#.
I fully understand why a server would add it on to a large family group with many small children.
For some reason, families are more apt to leave little or no tip. The bigger the family, the less the tip.
My coworkers and I have discussed this many times, and the prevailing theory is that they can't afford to go out to eat AND tip.
Anyway, there was probably signage, and you should have paid closer attention to the bill.