Jack and Mary had a daughter, Judy Baxter.

 Mary Carney, Judy Baxter and Raymond Baxter's story was first published in Angel-Headed Hipster by Steve Turner (1996).

Mary didn't want Jack to feel "obligated" because she was doing fine raising her daughters without any men.  Jack kept her letters forever and referenced Judy in Maggie Cassady with the line, "date with Judies."