Hitting the children with the Aravos:
The custom of the Chabad Rabbeim [as well as that of Tunisian and Morrocan Jewry] was to gently “whip” their sons with the Hoshanah branches. This included even their adult children. Children above age 18 received three light hits. Those below age 18 received one more “whip” than their age. Possibly, this custom is relevant to be followed also by the public, although was not traditionally followed by Chabad Chassidim [other than the Rabbeim]. Upon doing so, one blesses the children that they should know of no more pain throughout the year, and they should have both physical and spiritual joy.
 Sefer Hasichos 5705 p. 52; Igros Kodesh Rayatz [in letter to his daughters and son in-law] 15:67 “B”H, Hoshanah Raba 5690, My daughter Chanah, Chayah Mushkah, Shayna, and my son in-law Menachem Mendel, my grandson Shalom Dovber, Today is Hoshanah Raba and the custom is that the father hits the children with the Aravos, and blesses them that they should not know of any more smites throughout the year, and be happy spiritually and physically. I am now after Davening, and I did in my room as if you my children were standing next to me”; Otzer Minhagei Chabad 238; Reshimos Hayoman p. 266
Source for custom in non-Chabad literature: Nahagu Yisrael end of last chapter; Alei Hadas [Minhagei Algeria] p. 543; Or Shivas Hayamim p. 54 as custom of Morrocan Jewry; Sefer Moadim Lesimcha p. 446
Source of Minhagei Simcha on Hoshanah Raba: Sukkah 4:7
 See Alei Hadas [Minhagei Algeria] p. 543; Or Shivas Hayamim p. 54
 Sefer Hasichos ibid
 Implication of Rebbe Rayatz in letter to Rebbe where he writes “Today Hoshanah Raba the custom is that the father hits the children with the Hoshanos.”; Now, in general, when a Chabad Rebbe states that the custom is to do such and such, it becomes a public directive, or at least a custom followed by the public, although there are exceptions to this rule. One could however potentially argue, that the above letter was referring to the Rebbe Rayatz’s personal custom and not that of all general fathers, and hence explain why in the public talk he specified it as a custom of the Beis Harav [even though on this latter point one can argue he was referring to all the details of the custom and not the general custom]. It also remains to be understood why none of this was mentioned in Sefer Haminhagim, not even as a custom of Beis Harav, and perhaps the reason for this is because it is not specifically a Chabad practice. Indeed, we find in the customs of Algerian and Moroccan Jewry, that the fathers would bring home their Aravos and likely hit their family members with it, wishing them a good year. Whatever the case, I see nothing wrong with people adapting this custom, and it is similar to many other customs that were followed on the last day of Sukkos for the sake of joy, as brought in the Mishnah in Sukkos.
Does this custom apply to both sons and daughters? It is unclear from the wording found in the above sources if the Hebrew term sons and children is intended to be exclusive or inclusive of daughters. Certainly, however, the concept applies to children of both genders, and indeed in the letter of the Rebbe Rayatz, he includes his daughters, son in-law, and grandson.
 Reply of Rav Eli Landa in correspondence that he had never witnessed or even heard of this custom being followed by us, that the father whips his child with the Aravos. [As can likely be assumed, his father, Rav Yaakov Landau OBM, did not follow this custom at home, despite having possibly witnessed it by the Rebbe Rashab who did so to his son the Rebbe Rayatz. This further emphasizes the idea that the above matter was a select custom of the Rabbeim and not a directive to the public. Although, once again, the wording in the letter speaks to the contrary.]
 Igros Kodesh ibid