ETA for minor children, one of whom is our niece

ETA is not an issue - as long as you have their passport details, you can apply on behalf of someone else using the ETA web site:

"If you are applying for someone else who is not with you, you should apply online"

https://www.gov.uk/guidance/apply-for-an-electronic-travel-authorisation-eta

In terms of traveling with a minor who is not your own child, a letter from the parents is fine. Border Force may not ask for it, but if they do need to clarify your relationship it may be helpful.

Here's the official guidance:

https://www.gov.uk/government/publications/children-travelling-to-the-uk

Where you are more likely to need the notarized letter is when you show passports to leave the US. They may ask for proof that you have her parents' permission to take her out of the country.

Hi, Laura, I'm taking my two grandkids to London in about 3 weeks, and I applied for their ETA myself back in January. I had their passports and it was a very easy process.

I am also bringing a notarized letter from her parents that will cover the fact that they approve the kids leaving the country and also authorizing me to make emergency medical decisions for them if necessary.

That should be all you need. Have fun! When are you going to the UK?

Thanks so much everyone! This has been really helpful. And I'll add the note about medical decisions to our notarized letter. I'll do her ETA together with her in that case. We leave in early June. We are all so looking forward to it! Our niece has never left the country so this is big trip!

Thanks again,