5 C.F.R. § 843.409 – Rates of annuities.

(a) For each month, the amount of annuity payable to each surviving child under this subpart is—
1. (1) The difference between the basic child's annuity rate for that month and the total amount of child's insurance benefits under title II of the Social Security Act payable for that month to all children of the employee or retiree based on the total earnings (including any non-Federal wages or self employment subject to FICA taxes) of the employee or retiree;
2. (2) Divided by the total number of children entitled to annuity based on the service of that employee or retiree.
(b) On the death of the current spouse or the former spouse or termination of the annuity of a child, the annuity of any other child or children is recomputed and paid as though the spouse, former spouse, or child had not survived the former employee or Member.