There are a few rear bike seats that don't require a rack:
Yepp Maxi
Bobike Maxi
Britax (
http://www.amazon.com/Britax-USA-K05...itax+bike+seat)
There is also a Bulldog top tube seat (assuming you have a round top tube). Since the foot brackets on this angle down more, it might not interfere with your drops. Adeline Adeline in NYC used to carry it, but it looks like they might not anymore:
http://www.adelineadeline.com/kids/c...ing-guide.html
However, I would be very hesitant about clamping down the mounting brackets for any of these seats on a carbon frame. I would be concerned about damaging the frame. You would probably be better served by buying an inexpensive hybrid bike for riding with kids.