How about this: what would the lives of these children be like without that work? From your Western perspective, you judge this to be awful and evil, but what if the work they're doing is actually allowing them to make their families more than they could otherwise? What if it's the best option that currently exists? What if it contributes to increasing living standards in their country over time?[0] You can't look at a third world country through the lens of Western standards of living. A dollar a day is worth a great deal in some parts of the world.

The question isn't "why are some people poor", it's "why are some people rich". Human beings are born poor. We receive or make (or steal) whatever we have.

[0] Obviously, I am discounting political corruption that might keep wages artificially low.