A few years ago, I crossed from Canada into the US with the 16 year old daughter of friends of ours who was visiting us for a few weeks, to bring her to the next family she would stay with that summer.

We all made 100% certain that the situation was well understood, where we were going, etc., and she had a notarized letter from her parents detailing her travel plans. In the end, no unusual questions were asked, and nobody wanted to see the letter. Perhaps that's because we're all nice people and the border agents sensed that things were above board with nothing else going on.

Even when I flew to see some friends for a long weekend with one of our kids, I brought a signed letter from my spouse.

That being said, I totally support and understand the requirement, especially for international travel. Get a letter.