is the signature/ approval of their father required?
You have to prove sole responsibility.
Sole Responsibility (click)
319H of the Immigration Rules Part 8 is the applicable section of the rules.
319H. Requirements for entry clearance or leave to remain
To qualify for entry clearance or leave to remain under this route, an applicant must meet the requirements listed below. If the applicant meets these requirements, entry clearance or leave to remain will be granted. If the applicant does not meet these requirements, the application will be refused.
Requirements:
(a) The applicant must not fall for refusal under the general grounds for refusal, and if applying for leave to remain, must not be an illegal entrant.
(b) The applicant must be the child of a parent who has, or is at the same time being granted, valid entry clearance, leave to enter or remain, or indefinite leave to remain, as:
(i) a Relevant Points Based System Migrant or Appendix W Worker, or
(ii) the partner of a Relevant Points Based System Migrant or Appendix W Worker.
or who has obtained British citizenship having previously held indefinite leave to remain as above.
(c) The applicant must be under the age of 18 on the date the application is made, or if over 18 and applying for leave to remain, must have, or have last been granted, leave as the child of a Relevant Points Based System Migrant or Appendix W Worker or as the child of the parent who had leave under another category of these Rules and who has since been granted, or, is at the same time being granted, leave to remain as a Relevant Points Based System Migrant or Appendix W Worker.
(d) The applicant must not be married or in a civil partnership, must not have formed an independent family unit, and must not be leading an independent life and, if he is over the age of 16 on the date the application is made, he must provide the specified documents and information in paragraph 319H-SD to show that this requirement is met.
(e) The applicant must not intend to stay in the UK beyond any period of leave granted to the Relevant Points Based System Migrant or Appendix W Worker parent.
(f) Both of the applicant’s parents must either be lawfully present (other than as a visitor) in the UK, or being granted entry clearance or leave to remain (other than as a visitor) at the same time as the applicant or one parent must be lawfully present (other than as a visitor) in the UK and the other is being granted entry clearance or leave to remain (other than as a visitor) at the same time as the applicant, unless:
(i) The Relevant Points Based System Migrant or Appendix W Worker is the applicant’s sole surviving parent, or
(ii) The Relevant Points Based System Migrant or Appendix W Worker parent has and has had sole responsibility for the applicant’s upbringing, or
(iii) there are serious or compelling family or other considerations which would make it desirable not to refuse the application and suitable arrangements have been made in the UK for the applicant’s care.