You're misunderstanding me. The purpose of the citizenship clause of the 14th Amendment was to make the children of immigrants and slaves automatically United States citizens while excluding only select groups (including Indians living on reservations, as well as the children of diplomats and foreign belligerents). As the United States is not presently at war with Mexico, the children of Mexican nationals in United States territory are American citizens.
If being invaded by millions of people from a foreign country doesn't constitute war what does?