20 C.F.R. § 222.50 – When child dependency determinations are made.

(a) Dependency determination. One of the requirements for a child's annuity or for increasing an employee or spouse annuity under the social security overall minimum provision on the basis of the presence of a child in the family group is that the child be dependent upon the employee. The dependency requirements and the time when they must be met are explained in §§ 222.51 through 222.57.
(b) Related determinations. To prove a child's dependency, an applicant may be asked to show that at a specific time the child lived with the employee, that the child received contributions for his or her support from the employee, or that the employee provided at least one-half of the child's support. The terms “living with”, “contributing to support”, and “one-half support” are defined in §§ 222.58, 222.42, an 222.43. These determinations are required when—
1. (1) A natural child or legally adopted child of the employee is adopted by someone else; or
2. (2) The child claimant is the stepchild, grandchild, or equitably adopted child of the employee.