If they import they know that their product is selling and will make the problem of child labor worse. Because if they know that their product is selling they want more of it to sell and make a profit, which means that if they import goods made by children the children are going to lose their childhood to work.
Child labor is a crime against humanity. Developed counties should not buy products that are produced in this way. By purchasing them, we are supporting the system. Instead, we should impose sanction on countries which expoit children so that they will stop these practices. We should take a hard line against the exploitation of children.
There is a definite need to stop child labor, so banning the importing of goods produced from child labor will go a long way in doing this. Children need to be allowed to be children, not put to work as slaves to produce goods. Banning the goods will help put an end to child labor issues.
Most children are forced to work for the family. If countries ban child labor than more families are to be more poor, because children will have lesser money, and if they don't get a lot money doing their job now than will will probably won't have any money. Which forces parent to send them to do dangerous things. Joining gangs is on example.