A high sedimentation rate (ESR) only tells the doctor there is inflammation in the body, it doesn't diagnose the disorder.
Many children with rheumatoid arthritis (JIA) have gone on to play sports and have lived normal lives. The most important things are that she receives proper treatment now and your family becomes educated about the disorder. Since there are variations in types of JIA, that information should come from the healthcare providers as skyDancer suggests to insure the treatment she receives is specific to your child's condition. Ask them to provide you with all of the information they have and follow the prescribed regimen exactly. There is plenty of hope that she can win this battle if she is treated properly.