When introducing my kids to shooting, I used chamber reducers like Brownings Little Skeeter Tubes.
It allows you to shoot smaller gauge shells in a larger bore shotgun. The work great and don't really effect the patten much in the smaller gauge guns.

After starting with the 410, stepping up to 28 and ending at a factory 20ga; using reduced recoil 3/4oz loads they actually had less recoil than factory AA 28 gauge loads.
I duplicated the loads easily with my MEC but can't find and Hodgdon International Clays to make more.