I have to say the 391 too.
My daughter was on a youth trap team this year. Most of the kids, many girls included, shot the 391. In talking with a gun dealer at the state trap shoot he showed us a 12 ga 391 with a shorter stock for youth. It is otherwise a regular 12 ga. You can get a full sized stock later if need. Gun fit is going to be the most important facter in shooter comfort. You can get, or reload, light loads to help with recoil. Otherwise, I'm a fan of the 870 and O/U's. But, I think the proper fit and reduced recoil of the semi-auto are best for a new shooter. As far as safety goes, they can only load one at a time if that is all YOU give them!
Only since my daughter started shooting trap have I given more than 1 shell at a time. While she is essentially a full-grown woman at 17 I still reload light target loads, 7/8 oz @ 1100 fps. She shoots my old 870 and doesn't even notice the recoil. She's more than happy to burn through shells faster than I can reload them!
Make it fun for them and they'll be hooked for life!