The children are using crayons.

The correct answer is: They

A pronoun is a word that is used instead of a noun phrase. The pronoun they used when referring to a generic person whose gender is unknown or irrelevant to the context. The children are using crayons, here they can replace the children. They isa gender-neutral, plural pronoun, which means that it can be used to replace the girls just as easily as it can replace the boys, or even the girls and boys.