20 CFR § 404.736 - Evidence of a child's dependency.
(a) When evidence of a child's dependency is needed. If you apply for child's benefit's we may ask for evidence you were the insured person's dependent at a specific time - usually the time you applied or the time the insured died or became disabled. What evidence we ask for depends upon how you are related to the insured person.
(b) Natural or adopted child. If you are the insured person's natural or adopted child, we may ask for the following evidence:
(1) A signed statement by someone who knows the facts that confirms this relationship and which shows whether you were legally adopted by someone other than the insured. If you were adopted by someone else while the insured person was alive, but the adoption was annulled, we may ask for a certified copy of the annulment decree or other convincing evidence of the annulment.
(2) A signed statement by someone in a position to know showing when and where you lived with the insured and when and why you may have lived apart; and showing what contributions the insured made to your support and when and how they were made.
(c) Stepchild. If you are the insured person's stepchild, we will ask for the following evidence:
(2) A signed statement by someone in a position to know showing you received at least one-half of your support from the insured for the one-year period ending at one of the times mentioned in paragraph (a) of this section; and the income end support you had in this period from any other source.
(d) Grandchild or Stepgrandchild. If you are the insured person's grandchild or stepgrandchild, we will ask for evidence described in paragraph (c) of this section showing that you were living together with the insured and receiving one-half of your support from him or her for the year before the insured became entitled to benefits or to a period of disability, or died. We will also ask for evidence of your parent's death or disability.