How can expiation be made for the sin of using a sum of money (450 Kuwaiti Dinars) that was returned to her as a department secretary for another person's social insurance, and she spent it on herself and her children due to her dire need, and she cannot afford to repay it at present?

It is not permissible to dispose of entrusted money without permission, otherwise, it would be a betrayal of the trust. Your disposal of the trust for your personal benefit obligates you to guarantee it, along with the sin of betrayal. Therefore, you must repent to Allah and return the money to your workplace.