‘If Cruz’s parents split up, what country would hear any child custody claims?”
IIRC, under US uniform family codes adopted by most states, child custody jurisdiction is based on the residence of the child and/or custodial parent at the time of the custody filing. See the recent celebrity custody battle between Tom Cruz (coincidence) and his wife where she took the child from CA to NY to establish residence and file for custody there, IIRC.
You miss the point.
For instance, no United States Court, except in cases of child abuse perhaps, where the STATE would be a Party to remove the child -—
Well I very much doubt that if two Mexican illegal aliens divorced, any State Court would hear a custody case involving their 3 or 4 year old child.
They would have to go back to Mexico.