So the part that makes this guy a derp - he built what could have been a fully legal weapon, except that he didn't install the magazine lock correctly. What is a magazine lock? Its a device that lets politicians pretend that they're "doing something" when in reality it just makes them look stupid.
Mag locks serve no purpose in crime prevention because they don't have some sort of magic anti-criminal switch, they just annoy people trying to obey the law and criminals ignore the law so they don't bother.
Which, like "child proof caps" on medicine, they do a very poor job of, unless you're really rigorous in not letting them see how you get your self-defense guns out of the push button safe or whatever.
Plus or minus that age you have to also start "gun proofing" them, teach them how to handle guns safely, not to do it unsupervised, avoid the lure of the forbidden fruit, etc. etc. My father started with all of us at age 3, when he'd start taking us out for the safer types of hunting (not quail, where you're moving around a lot, albeit that would have exhausted us way too quickly). Don't remember when he started explicitly teaching it, but he did it by example starting then.
Maglocks do exactly zero to prevent this.
There are two things that prevent kids from shooting things/people they didn't intend to:
1. Parents
2. Training
Ignoring the issue is just bad, we figured that out a long time ago with sex education, not teaching your kids about the things you may or may not have in your house is dangerous.
Mag locks serve no purpose in crime prevention because they don't have some sort of magic anti-criminal switch, they just annoy people trying to obey the law and criminals ignore the law so they don't bother.