I believe that a demonstrative equivalent to "the" is not always necessary. In the lesson about animals, there are some examples: "The cat is looking for mice." = "Kočka honí miši." As I understand, the demonstrative may be dropped, if it is clear from the context which cat is meant (e.g. I only have one cat).
So here, after all, maybe it would be okay to translate "The children are growing fast." (as uttered, for example, by the grandparents on visit).