Assuming the literal meaning of “literal”, a child is, according to the OED, literally:
a young human being below the age of puberty or below the legal age of majority.
I’m not in any way defending child labor in general or Culvers in particular, but factually speaking, a 14-year-old fits between those two definitions (above the age of puberty but below the legal age of majority).
So that’s an inclusive “or” in the definition. If EITHER of those criteria are fulfilled, then the definition can be applied. Since the criterion about the age of majority is true then the definition is true.
So conversely, a person above the age of majority who hasn’t reached puberty yet (medical condition maybe? Just suspend disbelief for the sake of the argument) is still by definition a child.