My husband is a beach portrait photographer. I would say about 80% of the samples hanging in his studio feature families wearing white shirts and khakis, or white shirts and navy or denim bottoms. It just photographs so well, and makes for a beautiful portrait.
He had a family of 4 recently do a "costume" change. They wore white and khaki for 1/2 the pics, then the mom changed into a pink top, the dad into a green top, and the girls into matching dresses with both pink and green in them. Although the pink/green outfits were adorable, they chose the white/khaki look to blow up into 3 large canvas prints, b/c it just looked better.
Any white balance problems that the photgrapher might have during the shoot, can easily be fixed in photoshop in a matter of seconds!!
Good luck with the shoot! I'm sure you'll all look great!