Kirk's Challenge is a ST:TOS Kraith story by Eileen Roy. Artwork is by Connie Faddis. It was originally published in parts in the print zine Interphase vols 2, 3, and 4. (1975, 76, 77, 2016). The first section was reprinted in Kraith Collected #5.
Kirk, who, in Kraith, has been adopted into Spock's family, is gifted with two children from a previous dalliance: a ten-year old boy (Jai) and a three-year-old girl, (Embry.) Fatherhood was the furthest thing from Kirk's mind. And young Jai was not too happy about it, either.
All five of the original stories have been collected into one PDF document. There is no charge to download this collection of stories.