Hi there !
You didn't really mention any previous functional goals, but from the bit of info I'd say that improved posture is not bad alredy! We don't even know whether she walks or not... so I assume she does but got a wheelchair as well...
I'd look at whether her orthotic prescription needs updating (she would at least have a foot-drop I guess), as well as her walking aids (walker, crutches, canes, any?), the usual gait and balance training, riding a trike, how functional is her walking, is she better off with a good wc? Then wheelchair positioning and mobility, if you got a "whirl-a-wheel" that's great fun, then scooting on the mat, mat PNF program, getting on and off the wc on her own, then hydro, pulley and ball throwing (heavy ones) for UL, send for hippo for even better posture...
Then of course a good rapport with the parents, reasonable expectations on outcome (common goals), then the kid having fun with you during therapy and there you go!
OK, Regards,
Henner