Guidance for taking nieces to the United States

We are taking our two nieces to Orlando in August. I'm trying to get everything planned out well in advance. They both have their British passports already.

My brother-in-law and their mother were never married and he does not have any documents of parental responsibility so I wanted to get a letter from their mother to show to immigration officials/airline staff that states we have permission to travel with them. Does anybody know if there is a certain format for a letter such as this, and would it have to certified?

I also want to get a letter from her giving us permission to seek medical attention for the girls just in case (god forbid) something happens to them while we're there. Again, is there a certain format for this letter and would it have to be certified?

Naturally they will both have travel insurance.

Does anybody have a personal experience with a situation like this or can anybody point to me a site or sites that would help answer my questions?

Thanks in advance!

Whenever I needed letters like that when I was a kid traveling with on parent, we simply wrote something to the degree of:

To whom it may concern,

My daughter is traveling with her mother named..., they will be traveling for two months in etc etc, She has my permission to travel...etc etc..

Signed and dated.

We just typed it on the computer, printed it out and signed and dated it. Don't know you'll need something more official but this is what we used and we never had any problems.

I would have the letters notarized so they are proving that the parents actually signed them.That's what I did when my kids went to my sisters in another state.