I could bust one open in about 2 minutes with just about any tool. The thicker ones can be equally if not easier to break into for a thief.

As far as cousins or friends, what are they gonna do, go into his room, climb to the top of his closet, load the weapon and start shooting when he isn't there?

Like I said, in some areas of the country, guns are not any different than any other tool. Do you leave your chainsaw in the garage where your kid can get it? What about neighbors and cousins?

I think locking up guns is a great idea in some cases. I have safes. I use them, but a long gun up in the closet, unloaded, and out of sight is really NOT a big deal. There are 1000 other hazards in your house more likely to hurt your child that you probably aren't guarding against.