Depends on the piece of legislation you are referring to.
Section 2 of the Mental Health Act is about admitting for assessment with a 28 day time limit.
Section 2 of the Children Act 1989 is about parental responsibility
Section 2 of the Mental Capacity Act applies to those over sixteen and relates to their ability to make specific decisions.
Section 2 of PACE is around the police right to search.
All could apply to a child.