When my niece visited us a couple of summers ago, I ensured she had a notarized letter from her parents indicating their permission for her to cross borders.
When we crossed into Alaska, the border agent noted she was from a different location than I. He asked her a few questions. Where was she from? Where did she go to school? What grade was she in? What was her favourite subject? Why was she in Yukon? And for how long? How did she come up! How was she going back?
She answered them all on her own, and the agent didn't ask to see the letter. He waved us on.