Margaret Swynford (born c. 1369), became a nun at the prestigious Barking Abbey in 1377 with help from her future stepfather John of Gaunt, where she lived the religious life with her cousin Elizabeth Chaucer, daughter of the famous Geoffrey Chaucer and Katherine's sister Philippa de Roet.
Sir Thomas Swynford (1367–1432), born in Lincoln while his father Sir Hugh Swynford was away on a campaign with John of Gaunt in Castile fighting for Peter of Castile. However, thePeerage.com puts Thomas's birth later: "Sir Thomas Swynford1 b. 24 Feb 1372/73, d. 2 Apr 1432"; it cites Alison Weir, 'Britain's Royal Families: The Complete Genealogy' (London, U.K.: The Bodley Head, 1999), page 102.
Blanche Swynford, named after the Duchess of Lancaster and a godchild of John of Gaunt. (If, as suggested, she was born after 1375, this date is too late for her to have been fathered by Hugh Swynford, who died in 1371/2. However, since John of Gaunt obtained a dispensation for his marriage to Katherine for being Blanche Swynford's godfather, this theory can be discarded).
Although Thomas married twice, Hugh apparently had no grandchildren.