What do it matters? It is self-defence for Han anyway. Consider that, if someone talks about killing you, would you wait until they shoot first before you shoot back? Surely sometimes you do not have to wait until they shoot you before you shoot back, surely you can consider shooting first if you knows your life is at risk. For all we know, Han, even thought he do not have the Force in him, is actually experience enough to know Greedo is slowly pulling the trigger. To me, it is still self-defence if Han shoot first or shoot second.
What about cowboys? Cowboy A makes a draw for his gun, Cowboy B happens to be faster on the draw and shot A before A could get to fire first, it's still self-defence. Or should Cowboy B wait until A shoot first then B can fire back in self-defence unless he got shot, gasp his wound, grit hit teeth, spin around, fire at the ground or in the air in Hollywood style.
Isn't that what they meant by saying "Kill or be killed"?
And in my option, it do not matter in which order to watch Star Wars, do it 1 to 6 or original trilogy then second trilogy, or just let them watch only Star Wars (AKA A New Hope) on its own. There is no rules telling you in which order to watch, it's up to you. Just ask your kids which ones they want to watch first, most kids often don't bother with which order, all they care about is watching a cool movie.