At this point, without having seen the scene, I could envision how they could do a lot of it off screen without making it look harsh for kids, like you see Banner alone walking through the snowy tundra, then a close up to his waist reveals the gun in hand, it raises off camera and Banner whispers 'Betty' before you hear the bang.
The camera then changing to a pan showing the noise echo across the snow. Then a close up of the floor shows the bent bullet drop, and a large green foot steps over it with a roar.
I dont think most young kids would clock on to what was going on, but then again, I suppose its the concept of our would-be hero taking his own life, than the way they show it on screen, that Marvel wanted to steer away from.