Section 23. The department may act as guardian for a child under 18 years of age in its charge who has neither parent living, or of whom neither parent can be located, and for whom no guardian has been appointed. In such instances, the department may act with all the power and authority conferred by chapter 201, except that once a guardian is appointed, the powers herein conferred shall cease. The department may, without need for parent or guardian approval, serve as the fully authorized representative of a child in its charge not living with his parents or guardian for the purpose of completing and submitting an application on the child’s behalf to the division of medical assistance for benefits or assistance under chapter 118E or to the department of transitional assistance for benefits or assistance which said department is authorized to provide under chapter 18.