20 CFR § 404.762 - What is acceptable evidence of having a child in my care?
(b) If the child is living with someone else -
(1) Your signed statement showing with whom he or she is living and why he or she is living with someone else. We will also ask when he or she last lived with you and how long this separation will last, and what care and contributions you provide for the child;
(2) The signed statement of the one with whom the child is living showing what care you provide and the sources and amounts of support received for the child. If the child is in an institution, an official there should sign the statement. These statements are preferred evidence. If there is a court order or written agreement showing who has custody of the child, you may be asked to give us a copy; and