What happened to the 'little stranger'?

When we first see Bob Mitchell, it's 1918 and he's on a visit to his new next-door neighbors, Frank and Ethel Gibbons. He explains to them that his wife was too ill to accompany him on his welcome call, and he goes on to tell them that they're expecting "a little stranger." He also mentions that he and his wife already have a fourteen-year-old son.

The next time we see Bob Mitchell, it's 1924, and his son Billy is with him--now an adult and a sailor in the Royal Navy. They and the Gibbonses are gathered in front of their respective houses, preparing to depart for the British Empire Exhibition. As they leave, everyone waves goodbye to sickly Nora Mitchell. Never in the film do we actually see Nora or the younger Mitchell child, but Nora is mentioned at least three times. The last mention is of her death, something that apparently happens at some point in the mid-thirties. I guess we're supposed to infer that the child also died--at birth or very early in childhood. Otherwise, you'd think we would see the youngest Mitchell as a seven year old at the exhibition, a twelve year old attending Reg and Phyllis's wedding, an eighteen year old joining his older brother in the navy, etc.