is it bad to wear things made bye child labour  

We do have a problem here. If that child is helping to support his or her family, then by not buying something we may cause the family to starve. Then again we must not support slavery because many of these children have been sold by their parents to what they would consider their countries big businesses. So we should not buy product from a company in that poor country that forces children to work in unsanitary and dangerous conditions or does not grant them an education, plus proper food, and nourishment, nor should we buy from a company that will fire the adults and make the children work because they do not have to pay as high a wage.

However, if the child is an apprentice and is learning a trade from a master craftsman who has and still works for that company, and the boy and girl will eventually after learning it as well as get an education, then it would be all right to buy what that company sells.
And the company should make sure the working conditions are safe and the children only allowed to work certain hours.
So that would be the only condition. It worked in many European countries where children of poor families first became apprentices, then journeymen, and finally master craftsmen and it would work in the poor countries of Asia as well.