Originally Posted by
GUWonder
Many of my EU relatives' children with a different family name than the family name of one or both parents, yet they never have a problem when the trip is directly between the US and EU Schengen countries or UK-Ireland CTA. To play it safer than usual, some may get a notarized statement from the other custodial parent "authorizing" travel or a demonstration document indicating sole custody if applicable.
Second this. In any situation other than both parents with the minor the adult(s) should have such a letter anywhere with good law enforcement.