It's quite simple.
Grown men are expected to take additional risks to save the lives of children. This is an accepted cultural norm. I find it sad indeed that this must be explained in this age.
While yes, occasionally children become dangerous criminals or combatants, and yes, occasionally they must be killed, the idea that adults can wantonly shoot them without even attempting some other course of action is simply unmanly, and speaks volumes of american culture, which has turned cowardice into a superpower. The USA is a nation that any act of violence can be excused by saying 'I was in fear of my life.' Thus a coward has the right to kill, where the brave person does not. It is a norm that encourages cowardice, violence, distrust, racism, and dehumanization, all for the benefit of state power and gun lobby profits. So much for the land of the free and home of the brave.
And the worst part is that so many people don't even stop to think about it.
Also, another thing I shouldn't have to remind you of, but apparently do: suspect being reported as 'armed and dangerous' by a call in. Call ins are often not accurate, and the officer on the scene absolutely should be able to see the situation and make critical judgements rapidly and with an open mind, rather than acting according to preconceptions that may very well be false. As an example, recall that lad in Cincinatti, Tamir Rice, who was shot and killed as he played in a park with a toy gun, by an officer who had been told that there was a person 'with a gun' in the park. Because he had been primed with false information, and was a poor officer with many prejudices, tragedy ensued. A more observant and open minded officer might have seen and acted differently. This is why officers need to be trained to stop and LOOK, and SEE what is really there. Observation is a skill that can be trained. And yes, with training it can be done very quickly. That is what I am saying.
If we had well trained officers who stopped to think before grabbing their guns, we'd have fewer incidents, and more trust in the officers in the rare conditions where they did need to use force. Instead, we have ill trained yahoos going around pulling their guns first because THAT is what they are trained to do, instead of looking and thinking and actually defending the public. And we have people like you to thank for it.