The theory of Least Harm contradicts child labor. Children who are working are not able to get an appropriate education, and are at greater health risks. This creates harm not only for the child, but for society as a whole. Uneducated and unhealthy children, who then become uneducated and unhealthy adults, are likely to continue the cycle of poverty.
Child labor also goes against a Respect for Autonomy. "This principle states that an ethical theory should allow people to reign over themselves and to be able to make decisions that apply to their lives." We can not assume that children in the work force have reign over themselves or are making the decisions that apply to their lives. Child labor is the result of coercion or severe economic hardship and thus squashes free will.
Most of all, child labor goes against the Rights theory. This states that all members of a society have certain rights set forth by that society. In most societies around the globe, it has been clearly established that children have the right to childhood - a period of time in which they are nourished, educated, and given the proper conditions in which thrive. Child labor does not provide these conditions.