If proceedings are brought by the local authority (known as “care proceedings”), your child will automatically be a party to the proceedings. It is more unusual for a child to be made a party in private law proceedings (such as in disputes as to where a child should live or who the child should see).
If your child is a party to the Court proceedings, a Children’s Guardian will be appointed to represent them by CAFCASS who will instruct a solicitor to act for your child. The solicitor will usually take instructions from the Children’s Guardian. However if your child disagrees with their Children’s Guardian and the solicitor considers that they are competent to instruct them direct, that solicitor will then take instructions direct from your child. There is no specific age when this could happen but it would be very unusual for a child under 12 to demonstrate that they have sufficient understanding.