I bought the tristar youth 20 gauge auto and let a 9 year old girl shoot it, it was fine for her and recoil didn't bother her. But the youth stock was still too long for her. This particular shotgun came with an extra adult stock, and a 2 inch extension barrel, so it goes from 24 inch to 26 inch. Price was about 400 hundred or so.
I believe the SA's have less recoil than a O/U, depending how old your daughter is 20 gauge recoil may not matter in an O/U. Like you said be hard to find one in youth though.