I answered this question yesterday as well. I'm assuming you want a theological answer? Most people who are atheists/agnostics don't accept the answer or find it satisfactory, which I understand.
Most Christians believe that the evil in the world is the consequence of humans misusing their Free Will. When humans misused their Free Will, it brought everything evil into the world as a direct consequence, including seemingly mindless and needless suffering. That's one explanation.
Another is that, as humans, we simply cannot fully and truly understand God. We can try to, and learn about him, but he is far beyond us as he is God - a perfect divine being. The choices he makes, for example 'innocent people and children suffering' we might not be able to understand, but have to trust that he has our best interests at heart because he loves us.
I'll say again, this is all from a majority Christian perspective, and you will probably not find the answer satisfactory, which is fine. It's the theologically accepted answer, and has been for thousands of years.