Can my wife's child say if something happens to my wife?

Hi
My wife and child moved here in 2015 from America and we have gone through the midway spousal visa application without issue so, if all goes well, she should become a citizen in 2020/21.

Recently she was ill and wondered what would happen if she died, where would her daughters status be. She is 16 in Feb and wants to stay in the UK if anything did happen. She does have a father in the US she has contact with twice a year on holidays. Is there a legal document we would need to supply if her father demanded her return? What are her rights?

Also would she then have to go through citizenship?

Any advice would be greatly appreciated.

Thanks
David