I travel with my children and one thing that has started happening recently is that they will be asked at security checkpoints who they are traveling with, even when we're standing right there, the checking of minors' boarding passes has gotten much less perfunctory at some security checkpoints than it used to be. I think it would be prudent to have some sort of documentation provided because I have personally seen there is an increased emphasis on trafficking at security checkpoints. Maybe a notarized letter along with a copy of their passport (or their passport) if they have one to confirm their identity?