Matthias Doepke &  Fabrizio Zilibotti say international boycotts and sanctions against child labor actually exacerbate the problem:

In our analysis, we find that international interventions weaken domestic support for child-labour restrictions because they reduce competition between children and unskilled adult workers in the labour market. Unskilled workers then have less incentive to push for child-labour regulation.

When effective, trade sanctions or consumer boycotts move child workers from formal employment in the export sector to informal production, often in family-based agriculture. In the export sector, particularly in factories, children and adults perform similar tasks and therefore compete directly for jobs. In the informal sector, children and adults usually have different work responsibilities.

For example, on family farms children often specialise in tasks such as tending small animals, allowing adults to work in areas where they are most productive. Once adult and child labour become complementary in this way, restrictions on child labour no longer raise adult wages, which removes unskilled workers’ (and their unions’) incentives for supporting child-labour regulation.