The Frierdiker Rebbe spent Tishrei of 5690 (1929) in the United States of America, away from his wife and children who were in Europe. During that period of time, the Rebbe Rayatz sent numerous letters to his wife, Rebbetzin Nechama Dina, and to his children.
In a letter dated Hosha’ana Rabba 5690, and addressed to his children, the Rebbe Rayatz writes (Igros Kodesh Rayatz Vol. 15  p 67):
“…Today Hosha’ana Rabba, there is a custom that the father uses the Aravos [of Hosha’anos] to whip his children, and bless them that they should suffer no pain during the upcoming year, and be happy both physically and spiritually.
I am now after the morning prayers, and I imagined as if you, my children, are in front of me in my room…”
Three years later, on Hosha’ana Rabba 5693 (1932), a similar episode occured, as related by the Rebbe in his diaries (Vol. 14 p 8):
“On Hosha’ana Rabba, while I was standing in the dining room, the [Frierdiker] Rebbe came over to me and tapped my shoulder lightly [with the Aravos] three times and said: “Gevuros Mematkos” (the attribute of severity and restraint is sweetened [and transformed to Chesed (kindness)].”
When I asked him [the Frierdiker Rebbe] about it, he responded: “Until a child is 12 or 13, they get one whip for every year of their age plus one. Afterwards, they receive just three. This is how it always happened, every year during the day [of Hosha’ana Rabba] sometime after Shacharis.”