Where a 'prescribed child' is under 18, his/her share of the lump sum must be held in an interest bearing trust fund operated by the MRCC, until such time as the child turns 18. See 4.14 [2] for discussion of the procedure for arranging a trust fund and 4.15 [3] for the administration of that fund (including access by parents/guardians).
'Prescribed children' over 18 but under 25 are responsible for their own affairs and must be paid the lump sum directly i.e. not via a trust fund.
Links
[1] https://clik.dva.gov.au/user/login?destination=node/20653%23comment-form
[2] https://clik.dva.gov.au/military-compensation-srca-manuals-and-resources-library/death-handbook/ch-4-determining-srcaadb-lump-sum-entitlements/414-establishment-trust-funds-prescribed-children
[3] https://clik.dva.gov.au/military-compensation-srca-manuals-and-resources-library/death-handbook/ch-4-determining-srcaadb-lump-sum-entitlements/415-administration-trust-funds-prescribed-children