S.D. Codified Laws § 3-12C-103
For purposes of administering benefits pursuant to this chapter, the term, child, means an unmarried dependent child of the member, who has not passed the child's nineteenth birthday and each unmarried dependent child, who is totally and permanently disabled, either physically or mentally, regardless of the child's age, if the disability occurred before age nineteen. It includes a stepchild or a foster child who depends on the member for support and lives in the household of the member in a regular parent-child relationship. It also includes any child of the member conceived during the member's lifetime and born after the member's death.
For purposes of making beneficiary-type payments pursuant to this chapter, the term, child, means a person entitled to take as a child via intestate succession pursuant to the provisions of title 29A.
SDCL 3-12C-103