Fixed breech gun: Rem STS or GC hull, Green Dot or Hodgdon International, mint 3/4 oz CB wad, 3/4 oz Pb.
For gas gun cycling you'll need a slower powder burn rate. Suggest Alliant 20/28, Unique, or Universal Clays.
Recoil guns - whether inertia or long-recoil - are tough. You'll probably need 7/8 oz Pb.
There is abundant data in the Alliant and Hodgdon online data manuals.
3/4 oz is all I use in the 20, even at shoots, and have gone through 50k of them. Conservatively, very. But at 50#, your daughter may need further recoil reduction. Have a MEC 5/8 oz bar for really small kids. Green Dot is certainly OK, but I prefer Promo/Red Dot for this. Will not cycle an autoloader.
Messed with 1/2 oz 20s some, but velocities have been inconsistent.
