An immediate effect on a child who works under age is getting tired from working long hours and not getting an education or daily meals. And a long time effect on child labour is their income-earning potential as adults.
The reason why there are millions of children working as child labourers is because families or parent's who are too weak or too old can't work anymore to pay off their debts. So they "bond" a child. Bonding a child is giving your own child to work off the payments so eventually at the end debts will be paid.
To me child labour is out of hand. There should not be any more child labourers around the world period because education is our birth right, so against child labour we must fight.