Obviously, a larger labor force is good for the development of a country. Child labor is bad for the children involved, but it is good for the country they reside in. Children are willing to work for lower wages and they are more docile than adults who could organize and strike.
Child labor does not benefit the child or the child's parents. It does, however, often benefit the country's national economy. Many third-world countries are poor because they have dictators that abuse the country's natural and human resources. These dictators will continue to abuse human resources by allowing or requiring child labor until the UN puts a stop to it.