Is the divorced woman obligated to return the money she took from her husband without his knowledge, before their divorce, claiming that she knew he would not spend on their son, especially given that he indeed did not spend on their son after the divorce and that the father is the one who spends on him?

It is not permissible for a woman to take from her husband's money without his permission, because a Muslim's property is not lawful except with his willing consent. Whoever takes someone else's property must return it to him. However, if the husband refrains from spending on his wife or child, and she spends on them with the intention of recovering the money from him, there is no harm in that. The husband must be advised to fear Allah and spend on his child, for he is sinful if he neglects those he is responsible to support. The mother must raise her son to be dutiful to his father, and she must repent for transgressing against her husband's property.