A Samuel Williams, born 1900 in Carmarthen, is living in 1921 at St Peter’s in Carmarthen. There is also a Samuel Williams, born Llanelly, and in 1921 he is living in Llanelly Urban. It could also be another Samuel Williams, born 1901 and in 1921 living in Llanelly Rural. With the census being compiled early, if his birth date was after the census, it could be 1901. He may not have used the B. Without her birth year I cannot find such a common name, but they may be together. She could also be with one of her other children, or alone, or remarried. If you know her birth year, it could narrow the search.