I'm going with the 870 with light loads and a full length plug in it to make it a single shot. Even better would be a semi auto with a full length plug. It locks open after the shot and all they have to do is drop a round in the opening and press the bolt release.
Single shot break opens with a hammer are just not kid friendly, having to pull a hammer back that can easily slip with small, cold or wet hands, having to release a hammer if the shot isn't taken, heavy recoil even with light loads. 20ga are not pussy cats, I shot a 20ga sxs at an International skeet tournament when my 24gram 12ga loads didn't show up on time and even with target loads in the shotgun I hurt after 200 rounds.
