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Look at the 20 ga youth models.
Browning Micro Midas.Beautiful shotguns and sized appropriately.
10 gauge auto mag !
Lots to consider. LOP (length of pull) is most likely the most important. Her nose should be ~ 1.5 inches from the knuckle (closest to the wrist) on her thumb on the hand that holds the stock. A gunsmith can cut it down easy and also fit it with a nice recoil pad. Make sure the pitch of the stock is correct or the gun stock will continue slap her in the cheek and inflict pain to her cheek. Last is comb height. The eye is the rear sight on a shotgun and if the comb is too low or too high the POI (point of impact) of the shot string will be different than the POA (point of aim). Do her a big favor and take the gun to a place where you can pattern it to see how the recoil, and the POI is. A light gun will have more recoil. An O/U will most likely kick harder than an auto. Spend the money to get a fun gun for her to shoot and you will have a shooting partner for a long time.