What should the individual do to clear their conscience regarding a sum of money collected for the children of a deceased colleague, which was lost while in their keeping?

If the money is kept in a safe place and its safekeeping is not neglected, then the custodian (the depositary or agent) is not liable for guarantee, because he is a trustee and is only liable for guarantee in cases of transgression or negligence. The Prophet, peace and blessings be upon him, said: "There is no guarantee upon the trustee." If the money perishes without transgression or negligence, he is not liable for guarantee. However, if he transgresses or neglects, then he is liable for guarantee. If the amount of money is unknown, then an effort must be made to ascertain it, and action should be taken based on the preponderant opinion in estimating it.