A few things to consider - a semi-auto shotgun recoil is far less than a pump which is less than a single shot - so keep that in mind.
A 20 ga will do just fine at 40 if you pattern. My daughters used a Remington 1100 with 3" #4 copper shot with no problem at all. I even added a Tru-Glo sight to help them concentrate on the hit zone.
And we always tried to get the bird in at 20 yards or so as that is how I hunt.........
What a 12 does bring is more payload as far as number of shot per shell fired
........but a 20 will kill just as dead......
JW
Well said. I have shot some 20 guage pumps that kicked more than some 12's I have shot. With a decent semi-auto, they have very, very low recoil. That's what I'd recommend. Even a semi-auto 12 over a pump 20. If you went with a semi-auto 12, you could have your daughters shoot lighter loads and have less recoil than a 20 ga pump with more pellets sent down range. Just an idea.