Is it permissible for a mother to withdraw and manage funds allocated by the government for her son, which may not be spent for their intended purposes?

If the son owns the money and is allowed to dispose of it without a specific condition, then he may dispose of it as he wishes. As for a mother taking from her son's money, if he is an adult and gives her permission, there is no harm. If he is a minor who has not reached puberty, she is not allowed to take from it for herself or for his siblings. This is specific to the father, provided that it does not harm the son or take what he needs, and that he does not give it to another child. There is also an opinion that the father only takes what he needs. Based on the foregoing, the mother is not allowed to take from his money for herself or for any of his siblings.