Avoid anything with logos, graphics, characters, labels, etc. These tend to take the finished look of a professional portrait down a few notches, can be distracting (who wants people to first notice the Nike or Gap logo before the adorable little kid's smile?) and will date a photo quickly. Note: There are a few instances where a more stylized graphic on a shirt can look good if it fits the vibe of a photo.
If anyone is needing a trip to the salon, be sure to let the hair cut grow out a week or so in order to look most natural.
Don't make everyone wear all the same color; matching is boring and dated. Coordinate colors and looks, letting everyone have their own spin on the color palette (and every person does not-should not-have every color used in the color palette). And please no families all dressed in khaki pants, or all in denim and white shirts.
While trying to stay current and fashionable, do avoid obvious trends that will be dated soon. You can do fashion forward while still remaining timeless.
No bright white socks and no sneakers unless we're talking about something fashion forward and simple like Converse or Vans. And also be sure to remove watches or jewelry not complementary to the session's look