Look how coordinated this family looks! Nailed it huh?
And I thought I'd address something I've heard people say before regarding clothing for photos. I've heard don't wear white, and don't wear patterns. Well these guys did all that and it looks great. I think white in photos can be unflattering, and you might stand out too much in your group if they're all darker. If it looks good on you, and you blend reasonably well with your group, do it.
As far as patterns, nice solid stripes are good, polka dots are good, plaid is good. If it fits your personality it's good too (like some wacky shorts with hamburgers on them?). Really loud, bold patterns will stand out, so make sure it coordinates with your group. Kid clothes can be trickier since they're often so bold (my son is wearing tiger pants right now). If you're in doubt, stick to something more subtle or classic, but it definitely can be done.
My big don't is logos, or words on shirts. Subtle can be ok, but we're not shooting a commercial. The main problem clothing can cause is taking away from the people and faces, which is what we're really aiming for.