Why is child labour a problem? Simply put, it’s because it's allowed to happen. Companies making clothes would outsource the production overseas, where it is so much cheaper than doing it back home. On one hand, it's good for the world’s economy; on the other hand, bad chain management leads to terrible working conditions and child labour. Buying clothes is cheaper and easier than ever, the main way brands could make money now is through volume. The only way they could sustain that low cost, is through sweatshops overseas.
Then now we have a problem of low production cost, which means that the workers get less money than ever for all the work they have done. But when confronted by these issues, the fashion brand claims that they were shocked by that fact (classic denying), stating that while they did outsource the production with sweatshops and underage worker in mind. The blame then goes to middleman companies that promise the products to be shipped to the brands, the sweatshops are then under the contract of the middleman company to produce them. So the chain of production is terrible since the brands have no direct control over the production method (which, of course, could be solved easily, with more money, but well, that’s expensive). So they can’t enforce labour rights, companies continue to use these sweatshops and child labour continues to be a problem.