Briefly, child labour is usually understood to mean work that is either hazardous for children under the age of 18 or work that's done by children who are too young, generally under the age of 15.
You make a good point, and I think there's a broad consensus around forced labour. On the other hand, I would say that the voluntary standards we have internationally, especially the UN guiding principles for business in human rights, cover human rights generally. If you're going to ask companies to implement a due diligence policy, it seems to me that it's fairly easy to ask them to apply the steps of due diligence—identifying, preventing, mitigating—across a range of human rights abuses rather than solely limiting it to forced labour.
I'm curious about what Aidan has to say.