I'm guessing you probably have an idea as to what I mean by exploitation (though you may not agree with it) but I have no problem articulating it. Exploitation, in the labor sense, could generally be described as a unit of labor (worker) not earning the full value of their labor. In the United States, during the so-called Gilded Age, it was typical for children, as well as racial minorities, to make less per hour than more privileged groups, for the same exact work. Also, it's worth pointing out that a child who worked at a sweatshop 100 years ago, for example's sake, is technically a voluntary agreement, but not so much in practice. In most cases, children (12-15) entered the workforce not from their own volition, but because it was the only way for many of their families to get by financially, and public school was not fully universal.
As UKMikey suggests, a probable reason as to why child labor is outlawed is because of the lack of maturity and nuance among children to make such important decisions. It's for the same reason it is not permitted for underage children to drive, smoke or consume alcohol, have sex, or walk alone in some places. For someone who lacks this maturity and ability to reason fully, it can be particularly easy for them to be taken advantage of because of this, hence the "exploitation" part.
Lastly, I will remind you to be cognizant of the distinction between positive and negative liberty. I know this may come across as gatekeep-y, but to be truly libertarian, one should strive to achieve the greatest degree of both positive and negative liberty as possible. The two are not at odds with each other and can coexist in many instances. Undoing child labor protections would increase negative liberties in the sense that it frees the employer from external restraints on them, but decreases the positive freedom for the children, since they cannot fulfill their true potential if they are, in practice, forced to work 10-12 hour days. Their freedoms are infringed upon in both the extrinsic and intrinsic sense.