Is the deceased, who engaged in much backbiting, absolved of her sin through the forgiveness of those she backbit, after her death? And what is the ruling if she used to backbite small children?

Backbiting is forbidden and a major sin, and it involves two rights: a right of Allah Almighty, and a right of the backbitten person. If the backbitten person waives their right, the right of Allah Almighty remains, which is waived by repentance. If the backbiter dies before repenting, then their matter is with Allah. And people waiving their right after the backbiter's death benefits them, and the right of Allah Almighty remains, which it is hoped that Allah will forgive them for through the believers' seeking forgiveness and their prayers.