How so? Look up the definition of child. A fetus is included in it. All fetuses are children; not all children are fetuses. All children are humans; not all humans are children. That type of logic is something you should have been taught a long time ago.
noun
noun: child; plural noun: children
a young human being below the age of puberty or below the legal age of majority. "she'd been playing tennis since she was a child"
"I've known Kate since she was a child"
a son or daughter of any age. "when children leave home, parents can feel somewhat redundant"
an immature or irresponsible person. "she's such a child!"
a person who has little or no experience in a particular area. "he's a child in financial matters"
archaic the descendants of a family or people. "the children of Abraham"
a person regarded as the product of (a specified influence or environment). "a child of the Sixties"