I’ve been asked why is it that avelus for a child is only one month when it is a lot more painful than losing a parent where avelus is for a year. I suggested it might have to do with our obligation to respect parents but the mourner was not convinced. She is obviously associating the mourning period to the degree of pain.
In actuality both of you are correct. Losing a child is more painful than losing a parent, when a parent loses a child they will always feel the loss of the child, and the pain is a lot worse than the loss of a parent. On the other hand the main reason why we mourn is not just because of the pain that the mourner is in. It is a sign of commiserating with the deceased person, whose neshoma is undergoing difficulties in the transition from being in a physical body to living outside of a physical body, and also the din that the neshoma has to go through.
On the other hand the reason that we mourn a parent for 12 months is because of the mitzvah of Kibbud Av V’em (see Shach Y:D 344-9).
Another point, although losing a dear relative, and especially child is extremely painful, however chazal warn us not to mourn excessively, rather that we have to accept the din of shomayim (Y:D 394-1) and to strengthen ourselves in emuna, and understanding that although it is terribly painful, but still H-shem knows best ,and eventually we will also understand it.
May H-shem comfort all those who have suffered such terrible losses, and comfort us with the coming of moshiach.