Rei Ayanami -- the first suitable person selected by the
Marduke agency. She acts as the test subject for all experiments related to the
E Project, such as
Evangelion activation tests, etc. She participated in actual battles as the dedicated pilot of
Eva-00, and carried out numerous intercept operations against the
Angels.
Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.
The use of “Children” in the titles the First Children, Second Children, Third Children, is not a mistake. It should not be First Child, Second Child, Third Child; the use of “Child” is incorrect (no matter what AD Vision tries to tell you). GAINAX chose to use the English word “Children” as the identifier of the Evangelion pilots, it is not a translation mistake, it was intentional.