Someone entitled to an annuity for purposes of §§ 843.203 and 843.204 includes a child, even if the amount of the child's annuity is zero because the amount of the social security child survivor benefits exceeds the child survivor benefits payable under CSRS, unless -

(a) The child's annuity entitlement has terminated under § 843.408(b); or

(b) The child is -

(1) A disabled child under § 843.407,

(2) At least age 23, and

(3) Entitled to social security child survivor benefits in an amount that equals or exceeds the amount of the child survivor benefits payable under CSRS.