Is it permissible for the wife to spend from her children’s maintenance money on herself, given that she has wealthy relatives and that her ex-husband stated that her maintenance is no longer obligatory for him since her waiting period (iddah) has ended?

This money sent by your children's father is his property, and your possession of it does not make it yours. You must spend it only on your children. It is not permissible for you to take anything from it, especially since the father has explicitly stated that he does not permit it, as "the wealth of a Muslim person is not lawful [to take] except with his willing consent." If you are in need of financial support, it is obligatory upon your capable brothers, as some scholars hold.