Is it obligatory for an elderly person to understand the rituals of Hajj and Umrah before performing them, even if he is illiterate? And do the children sin if they do not teach their parents these rituals?

A Muslim must learn the rulings of his religion upon which his individual obligation (fard ayn) depends, for the Prophet, peace and blessings be upon him, said: "Seeking knowledge is an obligation upon every Muslim."

Therefore, the elderly man must learn the rulings of Hajj before embarking on it, in a way he can understand. His children must teach him and advise him to do so, as they are the most deserving of his advice. Their abandonment of this, despite their ability to do so, is a sin, because it is abandoning the disapproval of a wrong without changing it.