This one is less from me as a stylist and more from me as a mom.
The photographs I end up loving the most are almost never the perfectly posed ones where everyone is looking at the camera. They're the little moments in between: the ones where their personalities come through, or the ones that capture some tiny detail of who they were at that age. Years later, those are the photographs that bring back a memory or an inside joke or something I'd completely forgotten.
Once you remember that those are the photographs you're really there for, it's infinitely easier to stop worrying about whether everyone is smiling at the same time and simply enjoy being with your family.
And, usually, that's when you get the best photographs anyway.