The article features the speech genre “a message in a postcard” in Russian communicative practice of two periods: the period before 1917 Revolution and the Soviet period. The research is based on a subcorpus of 253 postcards written by children from the collection of postcards digital corpus “I am writing to you”. By observing the mastery of the genre canon by non-adult native speakers, we consider both the specifics of the speech genre and the dynamics of its changes that occurred at the turning point of two periods. The analysis showed that the speech genre “a message in a postcard” is sensitive to a large number of factors of different nature: its material substrate; achievements of scientific and technological progress; changes in the worldview and value picture of the world that occur in the linguistic and cultural community; the influence of dominant practices of using the visual component in communication. The changes brought by 1917 Revolution led to serious differences in the genre canon of the pre-revolutionary and the Soviet periods, which were reflected in children’s genre experiences. If in prerevolutionary children’s messages in postcards we observe vivid signs of natural written speech and even oral speech – ease, weak spelling control, phonetic principle of spelling, dialogical structuring, in postcards of Soviet children we see the use of speech templates, frequency of the collective addressee, strong spelling control, isomorphic intention of the text and visual design. Interpreting these differences, we conclude that before 1917, a message in a postcard was considered a kind of everyday family and friendly communication, close to its oral form, whereas in Soviet times the genre acquired a pronounced rhetorical public character. The sub-genre structure of analyzed speech genre also differs: in the pre-revolutionary collection, we find a variety of subgenres genres (“message”, “news”, “for memory”, “story about an event”, “from a trip”, etc.), and in Soviet times the genre repertoire narrowed to congratulations. Thus, a postcard written by children shows the genesis of the genre canon better, since the process of mastering the system of existing genre forms by an immature native speaker vividly profiles the specifics of these forms.