That depends on their visa.
A valid US passport and a plane ticket
Yes, a US citizen does not need a visa to travel to London for stays up to 6 months.
Yes, as a US citizen, you can work in London. However, you may need to obtain the necessary work visa or permit to legally work in the UK.
a resident?
A resident alien?
i am a us citizen i have been marry to Persian lady so how long it take for her to get citizen through marriage.
It makes no difference how long a US citizen of ANY age is out of the country. "Once a US Citizen, always a US Citizen." -UNLESS- you renounce your US citizenship and become a citizen of another country. However, since the question is about a 'minor' child that eventuality should not arise.
18yrs
You have to be a natural-born US citizen, so your entire life.
To become a representative you must be a citizen of the US for seven years.
Passport and airline ticket that's it. Before you land British authorities will have you fill out a form asking you how long you will be in the country, where you are going, and why you are there. It is pretty simple to do.
No, she is not. She is Pakistani, although she lives in England now.