From
www.gov.uk:
"The Home Office has an obligation under Section 55 of the Borders, Citizenship and Immigration Act 2009 to safeguard the welfare of children.
This means that if you apply as a child visitor we will want to know who your parents are, who you are travelling with and that there are adequate care arrangements for you in the UK.
If you are an adult (other than the parent or legal guardian) travelling with a child, you may be asked to present the parents’ or legal guardian’s written consent. If you are the child’s parent, you may be asked to provide evidence of the other parent’s consent if they are not travelling with you.
The Entry Clearance Officer may wish to see:
- the child’s birth certificate
- signed and dated letter from the child’s parent or guardian
- an official letter from an authority confirming guardianship"