The Official Solicitor is not used by local authorities in the way suggested. The Official Solicitor is an independent statutory office holder whose duties, when acting as a litigation friend, are to his client and not to any other party, be it local authority or the child who is the subject of proceedings.
The function and purpose of his office is to represent a person who lacks capacity to conduct proceedings, in civil and family cases, and proceedings in the Court of Protection, across England and Wales, where, either there is no other suitable person willing and able to act, or for some other reason, failure to do so would result in an injustice. The Official Solicitor usually becomes involved in existing proceedings because he is invited to do so by the court.