Generally, the law (at least in the UK) does not give parents an option to override age-restrictons relating to children accessing pornography. In practice, parents are able to show their children adult material that the parent has accessed; but parents may risk the attention of statutory child protection services if that is considered to be putting a child at risk of harm.
There is also the risk to the platforms which are accessed, as the law requires them to prevent children from seeing age-inappropriate content and there is no general exemption to say “except if the parents approve.” So a website cannot simply ask for parental consent and then allow a child to gain access to adult content without facing enforcement action by regulators.