Think about how your arrangement of people in a group family portrait can tell a story about the relationship between the different members.
A simple idea is to place the emphasis on the patriarch or matriarch of the family, or the newest arrival. By grouping the rest of the family around them, you'll be able to create a clear focal point.
For larger family group photos, use furniture - whether that's a sofa for indoor shots or a gate for outdoor portraits - to break the group up. Sit the children in front of it and have the adults standing behind it.