human beings of various ages, but all below age 18.
Zygotes, embryos, fetuses, newborns, toddlers, kindergartners, grade schoolers, middle schoolers, pre-teens and some early teens are all subsets of the term "children".
https://www.dictionary.com/browse/child
child
[ chahyld ]SHOW IPA
noun, plural chil·dren [chil-druhn].
a person between birth and puberty or full growth:
books for children.
a son or daughter:
All my children are married.
a baby or infant:
A child of six months can recognize family members.
a human fetus:
My sister miscarried with her first child at seven months.
a childish person:
He's such a child about money.
a descendant:
a child of an ancient breed.