Urine fasting isn't likely to help with scoliosis. Unless she suffers from some other serious chronic (internal) condition, it's not necessary.
Daily ingestion of urine will take care of minor health issues over time.
With regard to fasting and children in general, the best thing is to watch her appetite - feed her when she's hungry and don't force-feed her when she loses her appetite (for example, if she gets sick with a fever, etc.).