(a) A parent may designate a competent person, the Department of Family and Protective Services, or a licensed child-placing agency to serve as managing conservator of the child in an unrevoked or irrevocable affidavit of relinquishment of parental rights executed as provided by Chapter 161.
(b) The person, Department of Family and Protective Services, or agency designated to serve as managing conservator shall be appointed managing conservator unless the court finds that the appointment would not be in the best interest of the child.