The Da'as Zekeinim writes that it is Moshe talking to the parents who witnessed everything:
כי לא את בניכם. כלו' אינם ראוים ליענש ככם על המצות שהרי אתם ראיתם את מוסר ה' ואת גדלו ולא לפטור הבנים בא:
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. (Sefaria translation)
Because it was not your children - this means that they (i.e. the children) didn't deserve to be punished like you (the parents) over the Mitzvos. Since you (i.e. the parents) saw the morality of G-d and His greatness, whereas the children had not yet come (i.e. had not been born during this time).
Indeed, the Targum Yonason takes the same approach.
Also note the Malbim who writes expressly:
וידעתם היום אתם קבלתם המצות מצד הידיעה, כי לא את בניכם אשר לא ידעו ר"ל שא"א שיהיה לכם ידיעה ברורה באשר לא ראו בעיניהם כל הנסים שחשב פה ויאמינו כ"ז רק בקבלה אבל אתם עיניכם הרואות את כל מעשה ה' הגדול אשר עשה וא"כ ידעתם כ"ז מצד הידיעה:
And you know today, as you accepted the mitzvos from a place of knowledge, whereas your children did not know, which means to say, it is impossible for you to know with absolute clarity, that which they did not see with their own eyes, all the miracles he thought here and will believe. All of this is only accepted if your eyes have actually seen all the great deeds that Hashem did, and if so, you know all of this from the perspective of knowledge.
So the Malbim is essentially saying that if a person has seen firsthand the wondrous deeds of Hashem they know with absolute certainty about the Divine quality of Hashem's Torah and mitzvos. However, the next generation lacked this clarity as they were never zoche to see such wonders. This is why the onus is more on the parents.