I have 46 years of shooting hunting. Still have the stock sights on a few old 22.s . Even a full reciever sight and globe front on a rem 511 with a 25" barrel . Kill a small hill of small game and still today is used buy a grand kid to learn and still hunts with my children all with a 56 year old rifle. If some one can kill standard sights on a rifle they can do the same to a scope. It all goes along with learning about and respecting firearms. Brake it, you don't get to shot it.
Marlins have been around for many decades and open sights all work fine for all but a few that can break anything. But for a easier lto shoot longer sight radius that might pass for a more heavy duty sight put a set of Tech-Sights on the marlin and turn him loss on range targets.
For get the explosive targets as in some areas they will bring the cops. Get some reactive targets . Steel targets that swing. Self healing targets of many types can be enjoyable to shoot, even cheap grocery brand soda shack'n well work nicely. Maybe down the road scope the marlin and with a good ammo they are more than accurate and he can then learn to shot small tight groups. Save the scoped 06 till hes ready for some thumping.