The "rule" as per the website is one adult with the 4 year old son. GA will generally let the entire family board, sometimes even the extended family. The "real rule" is whatever the GA announces. The fewer famlies on the flight the more liberal the GA is likely to be.
Not to create an argument, but what else would define family boarding?
I have 4 people flying:
Me
Husband
Daughter 10
Son 4
We should board between A & B during the family boarding, as we are a family. If the rule is a parent and a child 4 and under, yes, you have a valid point, but from what I understand, it is family, so my husband and 10 year old, should board too.....now grandma, grandpa, aunt mary, uncle floyd...NO