Because child labor brings down the wages of adults. Child laborers who willingly work for pittances because they are in the main supported by their parents are like the semi-proletarianized peons who make less wage demands than the fully proletarianized because they still have access to land that allows subsistence agriculture. Let children work to produce use values for themselves (e.g., reading), not surplus values (e.g., flipping burgers, selling clothes at malls).
If children are to live fully independently of their parents, that's another story.
Yoshie