You don't ever plan to return to the U.S. at some point in the future -- even if only for a quick holiday visit (Thanksgiving, Christmas, Easter)? Why not just leave the pets with a trusted family member until then, and then be sure to book the holiday ticket with a carrier that will transport your pets?

Or, if you simply cannot be without the pets for a few weeks, how about simply using three of the four USAir tickets (e.g., hubby + two kids) and then buying a ticket for yourself to fly with the cats on another airline? You'll lose the value of the fourth ticket and you'll have to fly separately, but that wouldn't be the end of the world.