Actually, white is not a bad idea. White oxford shirts/turtlenecks on top and khakis or jeans for bottoms? (Same denim wash.) You really want people to be looking at your faces and general body language, not your clothes (is that a heresy for this site? lol)

I would choose something with a flattering cut and a flattering color.

There are really undless options- as long as it's a solid you should be ok. My friend is a photographer, and some of the looks that I have liked from her photo shoots of families have been:
white shirts with denim bottoms (or white dresses)
chambray/light blue tops with khaki bottoms
or pick two colors and mix & match (my kids did blue and black- one wore black jeans and a blue shirt, the other wore a black shirt and blue jeans)
You could also do a tonal look with one color- say blue. Everyone wears different shades of blue.

I've been working with a client who just did a photo shoot for professional purposes. It turned out really well!

o Wear a colour
o Create strong colour contrasts with what you’re wearing
o No small patterns

Personally, I prefer mis-matched clothing ensembles between the family members in family portraits. It looks more real that way, don’t you think?