While there is no indication that Sherlock Holmes had offspring (or even a true romantic interest) in the Canon, that hasn't stopped scores of authors, screenwriters, and other creators from imaginatively giving him some in latter-day works in all forms of media.
Aden McArdie (Chief Inspector Whitlock) previously played another Sherlock Holmes regular character, Lastrade, , in the short lived television series, "The Irregulars" (2021).
This isn't the first time David Thewlis has participated in a project where Sherlock Holmes partners with a young detective relative. He also played Grail in Enola Holmes 2.
About 17 minutes into the last episode, during a ball at the American ambassador's house, a US flag is displayed incorrectly with the field in the upper right.