In principle, as I recall the instructions, it is supposed to be the desire of the person who is named on the form but it obviously poses problems for children. Clearly parents have opted to answer on behalf of the child in some cases, so the result, the effect, has been probably a mixture of people who have chosen not to answer for their children and others who have.
Therefore, it's a mixture of the two scenarios. In some cases it really reflects the views of the person completing the form and in other cases it reflects the opinion of the individual who is directly concerned.