Although the son Vin isn't mentioned in the film, Mrs Miniver clearly is wearing miniature RAF wings on her dress when she visits the doctor in his office. Such a thing was common among relatives of those in the military, so this could be viewed as an unspoken reference to the unmentioned son.
The character of Vin, the oldest son of the Minivers is in the movie "Mrs. Miniver", but never mentioned in this film, even though they say at the beginning of the story where all the family members are in the world.