Is it permissible to issue a Sharia declaration for the the حضانة (custody/care) of children whose mother has died, without a judicial ruling?

If the person entitled to custody relinquishes it to another without a court order, then custody is a religious obligation (wajib shar'an) to protect the child from corruption or destruction. It is a communal obligation (kifa'iyah) if there are multiple custodians, and an individual obligation (ainiyah) if there is only one, or if there are multiple but they are not competent. It is a right of the custodian that transfers to the next in line if they refuse or are unable to fulfill it, without the need for a judicial ruling.