Paying poverty wages is wrong, and child labour is wrong. They don't cancel each other out.
"1. a young person especially between infancy and puberty
2. a person not yet of the age of majority..."
It depends on the definition of child you are using, I always thought the first to be correct.
Non-adult teens (“Teen” spans the border) are indeed children, and their labor – while in some circumstances, quantities, etc. – permitted, is regulated as part of the broader regulation of child labor (which is not a total ban, even for very young children, and is graduated by age.)
Whether it ought to be within the scope of acceptable child labor or not is a separate discussion, but it is child labor.
By prohibiting capable teens from work, either by pricing them out or law, we are hobbling their ability to learn valuable work skills. As a result we end up with adults that are lacking simple work skills that would make their labor more valuable.