well.......
I'm gonna get scorched for this, but here goes.
A twenty is not the equal of a 12, similarly loaded and choked. Period. A 3" mag 20 is the near equal of a std 12ga field load (1-1/4 oz of shot), but not a 3" mag 12 w/ up to 2 oz of shot.
The fact that your gun is a mod choke is working against it for turkeys. Turkeys are a head shot proposition and different from all other shotgunning. TIGHT chokes and sighting systems rule. Think "a short range rifle that shoots a gob of pellets".
Selecting a turkey gun for a youngster is a pickle. Not only is a 12 to much gun recoil wise, but it it often to much gun period, as in too heavy. And a 20 ga single bbl, with a load in it suitable for turkey, and properly choked full or tighter, will rattle a youngsters teeth.
There are two options: buy a dedicated youth gas operated 20, which the youngster will grow out of as a turkey gun, but could still be useful as a GP shotgun, OR a lightweight alloy/polymer 12 ga pump, loaded down with field loads, as in 1 0z, or 1-1/8 of shot, and a real turkey choke. The NOVA comes to mind. Shorten the butt on the 12 and add a space rubber recoil pad.
As the youngster matures, the 12 can be incrementally loaded up.
BAmaboy started w/ my 20" 12ga., loaded w/ 1-1/8 of shot. It was a bit heavy and he could not support if for extended periods of time as a 12 yr old. He was lanky enough that a factory length of pull worked OK.
Despite these stories of killing turkeys at 40 yds plus w/ a 20 ga. A full choked, heavily loaded 20 is a 35 yd and under turkey gun. Loaded w/ std lead pellets and an oz of shot, it is a 30 yd gun
The down loaded 12's are the same deal 30 yd or so turkey guns (assuming full or better chokes)