Favors and Voluntary Actions

Jane English argues that a system of favors does not apply to the child parent relationship, predominately because the actions that parents undertake for their child: raising them from birth, paying their college tuition, etc etc are done under the pretense of love. In other words, each of these actions are similar to a voluntary act of kindness. Therefore, the children are under no obligation to fulfill the parent's wishes. However, suppose the child asks for the favor. There is now a written promise binding the two parties, and the child is expected to fulfill his or her end of the bargain as well. Therefore in the case of paying for tuition, the parents may refuse to pay under the idea that they hope that their child will be able to become independent and pay for his or her own education. Then if the child were to ask for a favor of paying for the tuition, and the parents were to accept, would this not give the parents a say in what the child is studying? I would imagine that many of the interactions between parents and children resemble the above scenario. Suppose for example a child asks their parents to buy them a toy. This then gives them the obligation to repay their parents with something of similar or equal value. This could take the form of taking piano lessons or something similar. It was quite true that children are free to determine their destinies. However at the same time they oftentimes engage in such promises that give them an obligation to listen to their parents more.