Is it permissible to consider the monthly financial amount sent to the mother — who owns what suffices her — and which she spends on her married children who are experiencing financial hardship, as zakat on wealth?

It is not permissible to give Zakat to the mother because she is not among its rightful recipients. Her preparation of food does not justify giving her Zakat, as Zakat is given to the deserving as an act of ownership, not as food. Making food with Zakat does not constitute ownership for her. It is a condition for giving Zakat that ownership is transferred to the recipient, and according to the majority of scholars, it is not permissible to give its monetary value.