Travel with a minor who is not related

I'm Canadian and can't comment on your return to the US, but our 14 year old daughter went to Mexico with family friends a few years ago and we did as you've already suggested with a letter. We have a friend who is a lawyer who dealt with the notary, permission to travel, medical, etc.

https://help.cbp.gov/s/article/Article-3643?language=en_US

I did not dig into this, but it looks promising as to answering/addressing your issue

Yes by all means get 2 notarized letters, one with parent’s permission to travel and the other for you to seek medical care. We sometimes travel with our own granddaughters and need these 2 letters. The girls have been asked questions upon entering a country. Fortunately we’ve never needed the medical care letters but we have them.

Yes, get a notarized letter for both passport control and medical reasons. Hopefully you will not need it for the medical. You are almost guaranteed to need it at US passport control. As an added backstop you might want to have the parents on standby by phone when you cross the border. A friend's partner was travelling with her children. She was unavailable and US CBP pulled the father out of pre-op surgery to get his permission over the phone.

I would add that you should make sure ALL parents/guardians provide permission. I know when I led an exchange program we had this issue; we had to make sure that non-custodial parents also signed notarised letters. Divorced friends have to get notarised letters from the other parent to travel with their own kids. You might also want to make sure the medical insurance piece is covered.

In Germany, minors need a separate letter giving permission for swimming activities on things like school field trips, so make sure that if you think you might do things that would normally involve parent signatures / releases (horseback riding, water activities such as boating, snorkelling, or swimming, adventure type activities such as rope swings or mountain biking), get a letter that specifically mentions as many of these as you can think of that might be part of your itinerary.

It doesn't affect me, HowlinMad but I wonder if they would need permission for a Sommerrodelbahn? I distinctly remember losing about 8 inches of arm skin on one several years ago (some idiot on foot walked across the track directly in front of me).

I don't know, but I would include it just to be safe!

There are so much great advice already, I only have one thing to add and that is make sure you check on medical/travel insurance. I have taken my nieces, nephews, and friend's children a number of time to Italy. In advance of the trip, I arranged to get travel insurance (especially medical coverage). I fortunately never had to use it, but it was helpful to know that I had it available if I did.

Sandy