Is there a narration (hadith) stating that burying an infant with a deceased person alleviates the deceased person's torment or intercedes for them?

If a child is buried with his female relative in the same grave, scholars have considered it disliked or forbidden to bury two deceased individuals in one grave unless there is a necessity.

There is no evidence that burying a child with his female relative alleviates her torment or intercedes for her.

Perhaps the questioner is referring to what is authentically narrated in the Sahihayn, that a woman who sends forth three of her children will have them as a shield from the Hellfire, and likewise two. In one narration: "three who have not reached the age of accountability."

Al-Hafiz Ibn Hajar explained that this includes the permissibility of the promise, that Muslim children are in Paradise, and that whoever loses two children will have them as a shield from the Hellfire.

In the Hadith: "No Muslim whose three children die before reaching the age of accountability will fail to be admitted to Paradise by Allah, due to His mercy upon them."

It is reported in Sahih Muslim that the young children of Muslims are the butterflies of Paradise; they will meet their parents and take their hands until they admit them into Paradise.

These Ahadith include those whose children died before them, whether they were buried with them or not. However, there is no evidence for another person benefiting from someone else's children.