As I understand it, Marx advocated for children to continue working in factories, under capitalism, as useful for changing society. I agree that eventually socialism and communism will abolish work (wage-labour) and create factories very different to todays factories, but I dont think its relevant to what marx is talking about when he opposes banning child labour in e.g.

Critique of Gotha Programme:

A general prohibition of child labor is incompatible with the existence of large-scale industry and hence an empty, pious wish. Its realization -- if it were possible -- would be reactionary, since, with a strict regulation of the working time according to the different age groups and other safety measures for the protection of children, an early combination of productive labor with education is one of the most potent means for the transformation of present-day society.

Where he talks about the abolition of child labour as reactionary, even in 1870s germany. He isnt advocating children working just in the future communism, but as inevitable under capitalism and anyways beneficial for changing society