וִֽידַעְתֶּם֮ הַיּוֹם֒ כִּ֣י ׀ לֹ֣א אֶת־בְּנֵיכֶ֗ם אֲשֶׁ֤ר לֹֽא־יָדְעוּ֙ וַאֲשֶׁ֣ר לֹא־רָא֔וּ אֶת־מוּסַ֖ר יְהוָ֣ה אֱלֹהֵיכֶ֑ם אֶת־גָּדְל֕וֹ אֶת־יָדוֹ֙ הַחֲזָקָ֔ה וּזְרֹע֖וֹ הַנְּטוּיָֽה׃
Reconnaissez en ce jour, car ce n’est pas à vos enfants que je parle, eux qui ne connaissent pas, qui n’ont point vu, reconnaissez les enseignements de l’Éternel, votre Dieu, sa grandeur, sa main puissante et son bras étendu;
Rashi
וידעתם היום AND KNOW YE THIS DAY — Set your heart to know and comprehend, and to accept my reproof.
Chizkuni
ואשר לא ראו את מוסר, “and who have not witnessed Your chastisement;” Moses will furnish the details immediately.
Da'at Zekenim
כי לא את בניכם, “that it was not your children;” Moses warns his own generation that it was they who would be punished for ignoring the laws of the Torah, as it was they who had witnessed G–d’s miracles for them, not their children who had been born in the desert.