Safe?

As long as you leave out the percussion caps, black powder, smokeless powder, etc etc, it will be as safe as the bombs that're dropped all the time from RC planes. Matter of fact, I think AMA even gives points in scale competiton for "scale operations" that I think still include bomb drops.

Back to the safe side of this topic.........

I once took some "blown" eggs and opened the larger blow hole and filled 'em with powdered chalk. Glued on a note card fin and they looked like some of the concussion bombs that were dropped in WWII. Gave them to the RC power guys that were going to do a bomb drop event in their coming fun fly. Turns out they needed a bit of mass to get them to hit hard enough, but they worked good. I used marking chalk I got from a home improvement store. Nowadays it comes in colors. What'da'ya think a big cloud of red would do for 'ya.