My scout troop goes trap shooting every year. We have alot of small boys and a variety of shotguns. Almost all of them try the .410, but switch to a 20. The 20 gauge is much easier to hit with. Most of them miss with a .410. Then again, I've seen some really small boys go to town with a 12 gauge.
I'd also recommend the H&R Topper. Good gun, inexpensive. BTW: this is what most Boy Scout camps use.