Want a cheaper family ticket to somewhere? You need to have two adults, two kids. Win a competition for a trip/meal/tickets/whatever and if it's not just for two then it's for two adults, two children.
I know that they can't just do a family ticket saying "any and all family members welcome even if there's 16 of you!!" but really... not all families consist of two adults and two children. In fact, I'd imagine the majority don't.
There's plenty of families with no children at all but more than two adults, others with only one parent, others with only one child, or three or four children... (seriously, can you imagine winning a competition and having to tell one of your kids that they can't join in claiming the prize?)
If nothing else, couldn't it be widened to include one parent, three kids or something? Or just four people in general without any specification as to age?