A youth model 20g (most comp's will allow 20g and a youth stock will fit her better)
I'd also invest in one of those slip on recoil absorbing (PAST) shoulder pads.
http://www.amazon.com/Past-Mag-Plus-Recoil-Shield/dp/B0013R8J48
AND some low recoil 12g shells,someone else with better knowledge of the cartridges/shells available in 20g and avalible in North America can chime in from my own limited knowledge I'd go for Winchester AA featherlights,if they cycle ok.
http://www.midwayusa.com/product/2900611517/winchester-aa-featherlite-target-ammunition-20-gauge-2-3-4-7-8-oz-8-shot