Rufus Weylin is responsible for selling Sarah's children. Rufus is a slaveowner, and Sarah is his slave. It was a common practice to separate slave parents from their children. In his autobiography, to which I've pasted a link below, Frederick Douglass tells of being separated from his own mother when he was a baby. Here is what he says of his experience:
My mother and I were separated when I was but an infant--before I knew her as my mother. It is a common custom, in the part of Maryland from which I ran away, to part children from their mothers at a very early age. Frequently, before the child has reached its twelfth month, its mother is taken from it, and hired out on some farm a considerable distance off, and the child is placed under the care of an old woman, too old for field labor. For what this separation is done, I do not know, unless it be to hinder the development of the child's affection toward its mother, and to blunt and destroy the natural affection of the mother for the child. This is the inevitable result.
It was Margaret's idea to sell Sarah's children (not Rufus's as the other poster responded. Rufus was a very small boy when Sarah's sons were sold). The story tells that Margaret wanted money to buy new dishes etc. so she convinced TOM to sell Sarah's children. Interestingly, one of Sarah's son's was fathered by Tom's first father-in-law, so that slave was the half brother of Tom's first wife Hannah.