Is it even possible to waive a duty of care generally?
I do not know Australian law. I read the judgment of Lightfoot v Rockingham Wild Encounters Pty Ltd [2017] WACD 62 to suggest that at common law, the ability to waive of liability in negligence is uncontroversial. However, this freedom to contractually waive liability in negligence has been limited by state and commonwealth legislation to only allow exclusion of liability, in the context of consumer recreational activities, where the exclusion is limited to personal injury, and the waiver cannot shield the defendant from liability for reckless conduct.
Does the situation change when the waiver is on behalf of a minor?
I have read an article by the Honourable Justice Margaret McMurdo, "Dangerous Liaisons with the Civil Liability Act" (2003) that discusses whether a consent signed by a parent or guardian could waive liability on behalf of a child:
Where children are involved, a defendant service provider of
recreational services in Queensland may seek a signed consent by a parent or guardian, but even that may well not be binding on the child as it may not carry with it any power to act on behalf of the minor (see Homestake Gold of Australia Ltd v Peninsula Gold Pty Ltd (1996) 131 FLR 447, 456). Well-drafted contractual waivers of the type envisaged by s 68B TPA may therefore be binding on adult users of recreational services but perhaps not on children.
There is also skepticism about whether even an indemnity agreement could achieve the same result for the service provider:
recreational service providers may well endeavour to require parents
to sign an indemnity agreement in respect of minors taking part in recreational activities
under which the parents would indemnify the provider against damages or loss arising from a claim by the minor against the provider. He argues that it would be contrary to public policy
to deprive minors of their legal rights by this backdoor means and predicts that it would be unlikely that the courts would enforce such an agreement.
Conclusion
Unless there have been further developments of the law, it appears doubtful that an organization could rely on a guardian's purported waiver of the organization's duty of care towards a minor.