לדבר בם
The verse does not explain what and how one is supposed to speak about them.
Therefore, the verse means that one is supposed to converse in the language of the Torah (the Hebrew language) involving matters of Torah and Mitzvos. This means that it is a mitzvah to teach your children in a manner that they will be able to learn and understand the Torah.
As soon as a child begins to speak even if they cannot understand the words, one begins to train the child in the Hebrew language, and in the words of Torah (Shema and Torah Zivah).
Thus, it is a mitzvah to teach one’s children the Hebrew language, its grammar and verb formats. This is part of the method of appreciate and valuation of the Torah and Mitzvos. Lashon haKodesh, the holy language is the means to unlocking Torah knowledge.
Anyone that does not speak with their children regarding Hebrew is not able to teach them Torah properly. Therefore, it is fitting that leaders of the generation should spread the word that all parents should stress mitzvah of learning Hebrew just like all the other Mitzvos (such as Tzedaka or honoring one’s parents, etc.).
