In the good old days, women of all ages and sizes died from childbirth quite often. However, usually that was from infection; nowadays, we know to sterilize things that could cause infections.
There are plenty of complications that can cause death during childbirth. Most are rare, and many are treatable nowadays. (For example, failure to pass the afterbirth - nowadays, the doctors can look inside and scrape out any remaining bits.)
However, a 12yo will not have much more problems that an 15yo or smaller 17yo or tiny adult. Smaller-sized people have children all the time and it does not cause problems. Worst case, if the baby will not fit through the pelvis, the doctors do a cesarian delivery, where they slice open the stomach (sorry, the abdomen) from hip to hip to get the baby out. Then you end up with a scar that may or may not look too bad after a few years.
So yes it's possible, but not because she's 12.