I had my Handi Rifle pop open for the first time this past spring. I’ve had it now for 13 years, so I guess I’m lucky? I asked about it here, and got two bits of advise. Keeping the latch and shelf dry of oils, and make sure you close it with a bit of authority. Don’t slam it shut, just make sure it’s shut.
I did notice something a couple weeks ago that may be my problem. While cocking the hammer, my thumb slipped off and I pushed down on the latch lever by accident. I stopped myself, and didn’t shoot it. I picked up the rifle from it’s rest, and sure enough, it opened. So from this point forward I have to remember to do three things. Keep latch and shelf dry. Close it with some authority. And don’t let your thumb slip while cocking the hammer.
Maybe your son is not closing it right? Or maybe he is letting his thumb slip off the hammer and pressing the latch accidentally?