When my DD did this (also at age 8, she is now 14) she was out for 4 months. She had two pins placed during surgery (bottom of humerus was completely displaced) and re-location of ulnar nerve (new funny bone location), then casted shoulder to fingertip. Cast came off and pins out at 5 weeks, followed by two weeks of doing nothing with it - then physio for approx. 6 weeks (2X week) to regain range of motion. Cleared to full training at just after 4 months, when full range of motion was achieved. She was monitored every six months via xray and examination as there was growth plate involvement - when it became apparent (after 18 months or so) that her arm was continuing to grow normally, monitoring was stopped.
Other than a slight disfigurement (arm appears slightly crooked at elbow, due to the "elbow bump" protruding a bit more than it did before (purely cosmetic), her arm is as good as new. Regaining range of motion was definitely the most stressful part of the experience.
One of the most difficult things for my DD was that she thought (and so did we, or I should say, we didn't really even think about it) that when the cast came off, she would be good to go. In fact, when it came off, she couldn't even straighten her arm - hence the (at times painful) physio. From what I have seen/heard since then, this is very typical.
Hope this helps - Sending healing thoughts to your DD.