While child labor is viewed negatively, apparently child labor and child slavery aren't the same thing, and child labor though it could still be exploitative/cruel in other ways, can be done voluntarily by the child, and with fair treatment/compensation/etc.
I suppose you could make the argument that any child labor opens itself up to problems, but could it be done responsibly? And if not, then at what age do we draw the line of labor being not ok regardless of consent?
Nope. Children are not able to provide informed consent and thus cannot enter into contracts to sell their labor. Beyond that, there is a wealth of data demonstrating negative outcomes related to child labor, including educational underperformance, increased incidence of poverty, abuse, and crime, as well as the potential of workplace injuries to cause permanent developmental impairment.
There is no such thing as ethical child labor.
I tend to agree, but what about making a child do chores in a family household? Most children don't want to do it and some don't get anything in return, the tasks can sometimes be grueling. Would that always be unethical, or only when taken to an excessive degree that severely impacts the child?
Chores are largely part of childhood education. Humans need to be able to do things that they may not find fun to both to survive as functional adults and function as a part of society. They also help to teach responsibility and contribution to larger things than themselves, whether a family unit or society at large.