I shoot pistols with both eyes wide open, but my right eye takes full dominance. I'm RE/RH so I don't have the problem on this thread, (not to hijack the thread) but I used to always keep my left eye closed. Shooting with both eyes open is all about practice, and it's hard to practice something when you're not good at it. I had to make myself do it. It has actually become uncomfortable to close my left eye when I shoot, and I'm actually beginning to shoot long ranges with rifle with both eyes open.
It would be hard to explain to a 7 year old, but with practice you can put your focus through the dominate eye with both eyes open.
I know it's just a repeat, but I'd say pistols with both eyes open, and then lefty for rifles.