Yes a 410 can kill a turkey; of course you would want good shot placement, good heavy load to shoot, and a reasonable distance for the shot.
I can only tell you what I have learned from my experience. When I was a kid was I started off with a 410 like lots of other people have been. Since them days I have done a lot of hunting with a shotgun. I personally would not start a kid off with a 410, if I had my choice. To me the kids have a lot better chance of being successful and enjoying the experience with a 20 gauge than with a 410. I suggest start a kid with a good tripod; they usually are not strong enough to hold the gun up and be steady for a shot. This is what I did with my daughter. She was a pretty small kid when she started hunting at 10 years old. I had my daughter start with a 20 gauge. I had her shoot off of a tripod at a target with dove loads, and then moved up to pheasant loads. She shot her first turkey load when she killed her first turkey. She has been lucky enough to get three turkeys; and has been dove, quail, and pheasant hunting with her 20 gauge. If you look down this forum a little ways for the one titled "Recommendations for a Bird Taxidermist" there is a picture of her biggest turkey. Not telling you what to do, but maybe you know someone with a 20 gauge you can borrow that will work. Another great resource about hunting with a kid is NMPaul and others on here too. Good luck to you and nephew.
Thanks, SWB