If it were a dedicated indoor/basement/garage plinker, I'd go with the Airsoft. The BB's are also very cheap, and while it's a warranty "no-no" if they go into a soft trap, like a hung blanket with a scoop at the bottom made into a "J" in cross-section, they can be re-used.
They can be quite accurate. And with the "Hop up" which is an adjustable controlled backspin roller right at the chamber, (like a pitchers curve-ball but in the vertical plane to fight gravity) you can get them to shoot quite flat within 10-15 yards.
The downside to Airsoft is that it's almost all tactical/military replicas, or the cheap retail end mostly has the clear plastic/orange "toy" look. There's not much, if any in the way of traditional/target looking Airsoft out there. That might throw a wrench into any respect/safety lessons you're trying to teach. But I think it shouldn't matter much with a good calm kid.
Otherwise I agree with everyone else's recommendation to get a single cock spring piston bb rifle, like the Daisy Woodstock or Red-Ryder commemorative. They only shoot around 350fps or so, and can be shot indoors with a cardboard box full of rags for a target, with a loose blanket as a backstop as well.