870 in 20 gauge is a fine choice, its what i started my two boys out with now 11 and 12.
I was so impressed that i bought one for myself so much lighter and recoil is very low.
as for a 10 year old sitting still don't sell them short. last year my youngest had a jake 5 feet in front of him and never moved an inch until the bird walked away. why didn't he shoot you ask? there was a long beard just coming into the clear at 25 yards, the jake was actually in the way. the jake walked 10 yds to our left a he harvested a 21lb, 10.5 in beard bird and he did this all on his own. have confidence he will do fine.