Capitalists respond to legitimate threats to profit. Organizing a loud boycott that demonstrates the ability to move a concentrated amount of people in a condensed timeline at numbers large enough to be noticed statistically creates interest convergence, because the capitalist interest of preserving profit becomes threatened if they continue to do the behavior that the boycotters are protesting.

Thia is reformism though, and there's a major problem with reformism, which is that the boycotter boycotting chocolate candy due to child labor in the supply chain will only succeed in getting a specific candy bar company to choose to buy it's cocoa from somewhere else. This will not end child labor, and in fact, since it reduces the demand for child labor on cacao, it reduces the price of using that child labor for something else. In fact, given that most of the thousand food brands are actually owned by only 8 companies, they'll just shift the child labor pool to a different product line under the same company.