Child labor is most harmful to the mental well-being of a child. Though some forms of labor are physically harmful, all forms of child labor - by definition - deprive children of education and hold them back mentally.
Education is not a form of child labor. It is what children sacrifice when they are forced to give up school to work. It then becomes a vicious cycle as generations of children grow up without education, are unable to support their families, and have to send their own children to work to support the family.