I recently read a cool article about how to dress for family photos. The author Cheryl Lock suggested:
Keep Colors Simple: Avoid having everyone in different patterns and colors, as that could lead to a distracting look. Wear colors in the same range (darks, lights, pastels) and stick with solid or minimally patterned items.
Use the Location as a Guide: If you'll be shooting your family photo at a park, in the woods, or even in one room of your house, keep the backdrop in mind when you plan what your family will wear. For example, if you're heading to the woods, it could be cute for everyone to wear boots.
Keep the Kids Comfy: Taking family photos can be tiring, especially for little ones. So consider this: Is your 3-year-old more likely to sit still for an hour if she's in a soft cotton dress or in something more fancy and belted, with a stretchy fabric that scratches her? When it comes to kids, simple clothing is almost always better if you expect them to pay attention (and look good!) for long periods of time.
The same goes for hairstyles. Keep them simple and you're less likely to end up with kids who are messing with their hair.
Don't Go Too Matchy-Matchy: Although it might seem like a good idea to dress everyone in the family in the exact same outfit, keep in mind that doing so might erase some of the personality that makes family photos entertaining to look back on for years to come.
Skip the Trends: Even though your son loves his Transformers tee right now, if timeless is what you're going for, you might want something a little more classic for the photo.
I thought they were great suggestions. You can read the full article in parents magazine