הקהל את-העם ואנשים והנשים והטף וגרך אשר בשעריך למען ישמעו ולמען ילמדו ויראו את-יקוק אלקיכם ושמרו את-כל-דברי הורה הזאת: ובניהם אשר לא-ידעו ישמעו ולמדו ליראה את-יקוק אלקיכם כל-הימים אשר אתה חיים על-האדמה וגו’
Gather the nation, the men, the women, the taf, and the stranger in your gates. [This is] in order that you listen and in order that you learn and fear Hashem your G-d, and that you observe all the words of this Torah. And your children that don’t understand, they will hear and learn to fear Hashem your G-d, all the days that you are alive on the earth…
One of the last mitzvos described in the Torah is the mitzvah known as Hakhel. On the Sukkos following the Shemittah year, all Jews are commanded to come to the Temple and hear the King read from the book of Deuteronomy. The Torah says that this is so the people will learn to fear Hashem, and follow His commandments. The Torah stresses that all Jews are meant to be there, men, women, and children. The second verse clearly mentions children, and says they’re of an age where they don’t understand. The first verse, after mentioning men and women, says the “taf” are also meant to come. Who is this referring to?
Usually taf is translated as infants, those who are still nursing. However, some say that’s not the intent of the verse. The taf in the first verse are the same as “the children who do not know” in the second. Both verses are speaking about children who are approaching the age of chinuch, religious education. They’re too young to understand, but they’re almost there. That’s why the verse says they will learn to fear Hashem, because they’re still too young to do so. As opposed to the first verse, which simply says the people fear Hashem as a result of the mitzvah of Hakhel. The verse is saying that bringing these still maturing children also has some positive benefits.
However, our Sages explained the taf of this verse differently. They understood that the taf refers to infants, like it usually does. If so, what purpose is there in bringing them to hear the King read from the Torah? They have no clue what’s going on. They say it’s solely to give reward to those who bring them. It’s true that taf can mean those approaching the age of chinuch, as we see from elsewhere. However, no one can disagree the word also can refer to infants. Further, the fact that the two verses use two different terms indicates they’re referring to two types of children: infants, and children who are approaching the age of chinuch. Since taf can refer to both, the first verse includes both. However, the second verse is only referring to older children.
The verse tells us the purpose in bringing the older children, those who are approaching the age of chinuch. Their parents should habituate them and educate them in the fear of Hashem. However, the infants, who are included in the first verse with the word taf, what purpose is there in bringing them? They themselves are too young to know what is going on to benefit from Hakhel. Their mothers don’t need to be told to bring them, because any mother would bring their babies without being asked. Who else would watch them? Who else would nurse them? How could she not strain herself to shlep her baby all the way to Jerusalem to the Temple?
It is this question that our Sages came to address. Why were the Jewish people commanded to bring their infants to the Temple for Hakhel, they would bring them anyways? Why did the verse use the word taf, which includes infants? To this they answer that it was solely to give reward to those who brought them. She would have done it regardless. Nevertheless, Hashem in His infinite kindness gave reward to those who would struggle with the journey anyways. He does in many instances throughout our lives. It’s something we should recognize and be appreciative of.
 Based on HaKesav VeHaKabbalah to Deuteronomy 31:13
 Deuteronomy 31:12,13
 Sefer HaMitzvos Aseh #16; Sefer HaChinuch § 612
 Rashi to v. 10, quoting Rosh Hashanah 12b
 The men are already meant to be in the area for the festival of Sukkos, like the other pilgrimage festivals (see Deuteronomy 16:16, Sefer HaMitzvos Aseh #53, Sefer HaChinuch § 489)
 V. 10
 Ramban ad. loc.
 Chagigah 3a
 See Exodus 12:37, which separates between גברים (which is included in איש) and טף. The Ramban to ibid 30:12 says איש is thirteen, and טף is less than that. Cf. Rashi to ibid 12:37, who understands the גברים of that verse as twenty and up. That makes the word טף in that verse difficult. See Mizrachi ad. loc.
 As we can see from Numbers 31:18, and 16:27 (see Rashi ad. loc.)
 HaKesav VeHaKabbalah brings from the sefer Revid HaZahav by Rav Dov Tarivish, the Av Beis Din in Vilna in the generation of the Vilna Gaon, who brings that the Yerushalmi Chagigah 1:1 understands the taf of the verse to be referring to infants. The former also proves this from Megillah 5a