Why did people in the past say "no children under their knees" when they had no sons?

Children depend on their parents' knees when they are young, so "below the knee" is childhood. Later I borrowed it from my parents, which means kindness. "The book of filial piety. Three Records: "So, I am at my own knee." Zheng Xuan's note: "Below the knee, it means when the child is young." Later used to express love for parents; And it is used as a tribute when communicating with parents. Yuwen Hu's "Report to Mother": "Knee violation, thirty-five years." This means that I have been away from my mother for 35 years.