Simply call customer service. You won't be able to do this hairy account tweaking with family plans on the web sites such as Amazon or Wirefly. As long as you don't have any unpaid ATT bills and your credit is good it won't be much of an issue. Actually, it's even easier now since you are on month to month service. You'll have access to free phones for the new add'tl lines. Keep in mind that in order to be eligible for the rebates on wirefly/amazon you will have to make certain you don't make *ANY* changes to your plan. That can be tricky with new lines, combined accounts. I'd opt for going the safe route any obtaining phones through ATT, although they may cost a bit more. You might look and see what Wal Mart can offer you price wise, as it might be quite close to what Wirefly or Amazon can do for you. Keep in mind combining accounts will require advanced help from customer service so that's why I recommend you go through them first.
For more specifics you could get a second opinion over @ howardforums.com

-Pedro

p.s. Don't forget some of the pricing depending on handset may also require you sign up for 2 years worth of data plans for each line with a PDA or smart phone. That can eat into your savings.