Why had Charles sneaked their cow down the mountain to the Stevens' place?

Lyddie and Charles run the farm in their father's absence. Their mother's mental health is precarious, and she is not much help around the farm. Lyddie and Charlie take it upon themselves to breed the cow each year as their father had done when he still lived with them. For breeding, they make use of their neighbor Mr. Stevens' bull. Because of some odd ideas their mother has, she has told them not to associate with the Stevenses. The Stevenses are Quakers, and their mother's religious prejudices cause her to think that they are "heathens" and that to associate with them would be to "have truck with the devil." In addition, the Stevenses are abolitionists, and Lyddie's mother doesn't approve of them for that reason as well. So Lyddie and Charlie don't let their mother know that they are breeding their cow with the Stevenses' bull. Selling the calves each year is one of the few sources of income the family has, and the children know, even if their mother doesn't, that the family would not be able to survive without the cash they earn from selling the calves.