Intestacy - Priority to apply

Deceased has died intestate leaving 3 children, 1 is an adult 2 are minors. A minority interest arises due to a pension lump sum forming part of the estate. Adult child is willing and prepared to apply for Letterso of Administration. The two minors have 2 different fathers who have PR for those minors. Can the adult child apply with the deceased’s mother as the adult child or do we have to involve the fathers of the minor children. Preference of adult child is to apply with his grandmother (the mother of the deceased). Thank you in advance