Depends on when you end up boarding. If you are OK with the back of the plane, then you're probably good with a B boarding pass. However, if you can't check in at 24 hours, there are a lot of people on the early-bird check in or a lot of those who CAN make it to check in at 24 hours and you end up with C passes, you might end up scattered. I don't think I would really count on other people who have already boarded giving up their seats so your family can sit together.
I have this problem but I am a solo flyer and my only desire is to not end up in a middle seat, so I'm going to do it without the early bird check in, but depending on how I feel later I might just change my mind and sign up for it.