Hmmmm
Lot of the 20s are lighter & will point more naturally due to thier weight - easier swing, etc.
"Problem," if any will come into play regards the recoil of the loads used. A 20, all equal (& assuming lighter weight of the 20), will recoil "harder" than the 12.
Fit makes a big difference, 'course, that'll matter too far as pointability & actually hitting stuf - the whole point of the exercize.
The Wife's 100 lbs/5' & shoots a 870 Express Youth 20 - but really, she sucks at it, but that more to do with SG trigger time than anything else. She's a wonder with DA revolvers & bolt rifles .....
Try a lightweight 20, built to fit & some light loads - first off on stationary targets = coupla clays sitting on a bank. Go slow & work on the acquire, mount, aim, & trigger release.
Go from there. Allow it to be gentle to work her into it. This is no stage for the "no pain, no gain" bit. You want her to have fun.
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& thanks. Anybody teaching young lasses to shoot is OK in my book.