I know I will probably have to talk to a lawyer about this but hoping someone has some knowledge of where to start.
I have live in USA since 2003 with my daughter (born 1999). I became a US citizen last year, and we are in the process of getting US passports. I went that route before getting her N600 certificate because I thought it would be faster and we are going to UK for a trip in 2 months.

So.... issue has arisen that I need to prove custody so she can get her passport. Under the UK law at the time of her birth, I had sole custody because I was unmarried. As a result, I don't have a piece of paper to prove that, it was simply the law. In all my 13 years of USCIS interviews, visas and forms for my daughter and I, this was never an issue. Frustrating to deal with it now from another authority.

The passport office has requested one of the following from my daughter:
- a copy of her US Certificate of Citizenship (didn't get yet because I thought this would be faster)
- a copy of custody papers (not applicable)
- or "statement from a competent authority (such as a court or state agency) certifying that your state or country of birth presumes that you are in the legal custody of your U.S. citizen parent. A letter from an attorney citing the statute is also acceptable if accompanied by a photocopy of the statute."

Q1. The last option is my only option, but I am not sure where to begin. Anyone have any clue? The British embassy? US lawyer? UK lawyer?

Q2. If we don't get this resolved in time, is there any issue with her traveling on her UK passport as she has 6 or 7 times in the past 13 years?