Most of these countries have large numbers of children and fewer numbers of adults. Because of this disparity, children make up most of the labor force. There is such a high demand for workers and such a low supply of jobs that kids are pulled into the market. Because most people in third world countries are poor there is a lot of demand for work. This high demand for workers also pulls the wages that companies must pay down.
Children are even cheaper because they are so easily controled.
The fact that kids will not usually rally to form unions or go on strike is one reason why child labor persists. If nobody is there to stick up for the children, then nothing will be done about the problem. Both governments and parents are either unwilling or unable to stand up for the children. Governments are unwilling or unable to stop Child Labor practices because Child labor can sometimes be a large part of that country's economy, especially in third world countries. In fact, Child Labor is more likely to happen in third world countries than other countries, because these countries are more likely to turn a blind eye to the practice.