Sorry to gatecrash!
Just saw this - don't panic is the first thing! I'm sorry this has happened, it must've been really scary.
My brother was diagnosed with another type of rheumatioid arthritis - it was juvenile onset rather than juvenile. The difference is that the one my brother has comes on in adolescence and then stays through adulthood, whereas juvenile rheumatoid arthritis usually goes with age. It's not quite the same as the arthritis we think of that comes with old age, and it can completely be managed. Was she prescribed any medication?
I know it's hard, but please try to stay positive. My brother was diagnosed with his form of it, which is much more aggressive than the sort it sounds like your daughter has, at 14 and told he'd be in a wheelchair by 16. He's not, and he's even coming off his methetrixate medication. It's NOT the end of the world to hear the diagnosis, it's a good thing it's been caught early, which it sounds like it has, because now she can get the trreatments she needs. Physio was amazing for my brother, as was hydrotherapy because it affected his knees really badly. There might even be support groups in your area with other mums going through the same thing. You're definitely not on your own!
It's a good sign that she's only got it in one place, the best thing you can do is get as much info from doctors/specialists as you can and make sure she keeps moving, it's better to move it than keep it still!
I really hope you and your little girl are ok,
Em x x x