My 12-year-old is likely going through the same thing (X-rays on Friday). Hers sounds fairly mild compared to the two girls at your gym... only has pain when she does something that requires her to overextend her back (like most tumbling, but no pain running or doing bars or even, apparently, vault), no other pain or numbness, etc. but she does have pain in 3 vertebrae in her lower back. I know our gym has more strength-training than most... a kenesiologist from a local university comes in 2x a week to do "core" training with them, in addition to normal strength training. Clearly 12 is a time of rapid growth and body change, as well as the fact that they are doing skills that are more taxing on their bodies (my DD is a level 8). It may just be a "perfect storm" of stuff going on at once.
Can you tell me what the prognosis is for the two girls as far as continuing in gymnastics?