If not working in a mine, these children will be working subsistence agriculture, which is equally as back-braking. The underlying problem is an excess of manual labor in Africa, caused by excessive birth rates. Take fertility rates down to sub-replacement like they are in the rest of the world, and mining will just be automated.
In what fucking world holy shit. Good lord, seriously. Labor IS PEOPLE. There is no such thing as an excess person.
The underlying problem is an economic system that allows/depends on the exploitation of vulnerable and precarious labor. If you're going to justify this then grow a spine and do it, don't deflect it back on the people being hurt the most saying they should cull themselves for our moral convenience.