Would it be wrong to argue that the child is a slave, until he is conscious enough to be able to legally consent?
First, what is a slave? Humans treated as property that can be bought, sold and disposed of at the will of the owner. Slaves have no rights; they don’t own their lives or anything else.
A child can’t legally consent, but their parents are morally and legally empowered to consent on their behalf. For example, a child can’t sign a release for surgery; only their parents can. Parents act as guardians, making decisions on behalf of their children that they think are in the children’s best interest. However, parents are not empowered to violate their children’s rights or to treat them as property or slaves.
I’ve been hearing a lot of arguments against child labor, that children of young ages can’t consent - therefore, if any child were to work somewhere, it would be slavery
Children are effectively wards under the care of their parents. They can legally and morally be told what to do when those actions are in their long-term best interest – which excludes things that physically or mentally hurt the child, of course.
With regard to child labor, no rational parent would want their young kids to work. However, if it’s a choice between working and starvation or death, then it seems clear to me that work is the better choice. In fact, if a parent let their child die when work could have saved them, I would consider that to be child abuse, not the other way around. As long as the parent’s consent is involved, it’s clearly not slavery. The child is simply being directed to act in the child’s long-term best interest.
OTOH, if a child was working without their parent’s consent, at the call of and under the control of some third party, it could easily start to fall into the category of slavery – and even if it wasn’t quite slavery, it shouldn’t be legal, either.