Wills, probate, letters of administration, powers of attorney
You could ask his last employer the name of his superannuation fund as this is where they will be sending his compulsory contributions. But you say they refused the request, perhaps an application to the Superannuation Complains Tribunal may help resolve this. See SCT for details.
However be aware that a superannuation fund will only release the money according to law, this means to the persons currently nominated or to his estate.
His nomination to pay it to your children may have expired if he had not renewed it. The only way you are going to find out is to find the fund and then write to them.
citizen-joe 2018-10-19 23:20:03