I'm not up on the specifics of this case, but I can address the issue of keeping guns out of the hands of kids.
If someone really wants a handgun (even someone prohibited form possessing it like a felon or a minor) they will eventually end up getting one. And it would be difficult to effectively prevent it without creating a system that would hinder nearly all other legitimate gun owners. The best (read: most effective) way I could think to do it would be to treat it like NFA weapons. Register the gun and then make it a felony to give the gun to anyone else without proper documentation/paperwork and taxes etc. And even that wouldn't completely eliminate the problem, it would just make it more expensive for the buyers.
You don't even have to hide it. Not all arms deals take place on abandoned piers at midnight
Right after I turned 18 I bought several pistols in FTF transactions from people at gun shows and off forums. out of maybe 3 or 4 purchases, I was only asked for ID once. I could have gotten those guns a year or two earlier (illegally) and no one would have been the wiser. Now if we're talking about young kids, I agree that this isn't really plausible. But younger kids have friends who are older who may be able to look like they're legal.
To put it another way, drugs are essentially completely illegal for anyone in the US to use, possess, or manufacture (lets forget about medical marijuana for the sake of this example), but it would be nearly impossible to find any city in the US that didn't have at least some form of drug trade. Taking that one step further, lots of kids can get drugs if they want them through friends or friends of friends etc.
Now take an item (like a gun) that is completely legal for the vast majority of people in the US to have, possess, carry, and trade as they please without any regulation. Is it any surprise that a dedicated kid could get his hands on a gun?