Why do children suffer or enjoy in the material creation?

We forget, or are not properly educated to see, that the "jiva soul" in a child's material body is eternal without beginning or end as Bhagavad Gita As It teaches us -

Chapter 2 text 20

"For the soul there is never birth nor death. Nor, having once been, does he ever cease to be. He is unborn, eternal, ever-existing, undying and primeval. He is not slain when the body is slain".

And in fact, the jiva soul in a child's material body has had millions of previous bodies (lives) 

This means they are therefore responsible for whatever pious or impious actions they did in all those previous lives.

This why one child is in a rich family and is always healthy, and why another child lives in poverty or is always suffering from different diseases.

It is all based on what the jiva soul has done in previous lives.