In reply to Sleepymrsp:
My small 7 12 year old uses the mammut kids sit harness. She is small for her age too (118cm tall 3 stone 3 pounds). The mammut one goes small enough for her tiny waist and she regularly uses autobelays and doesn't turn upside down.
I have found I can get her to turn upside down if the tie in loop is big enough when I'm belaying her but we usually ties herself in and it isn't a problem. We found her full body harness (the wild country one) stopped her climbing overhangs.
Where do you climb? Do they have various harnesses you can try? An alpine might work too as the tie in loop is higher up and she will never grow out of it!