But what is "effective" in this case? My kids use huge Whatsapp groups as a form of social media, will Whatsapp be banned? Using a VPN to access TikTok from outside Australia is not beyond the skills of a determined kid, will VPNs be banned?

Those details matter to say whether or not this is a good idea, why not wait until you have the answer to those questions before enacting the law? (I think there are no good answers too this, and it's probably better to say this is a job for the parent, not for the government)

But that’s my point exactly. You saying “I only support the law if it’s implemented ineffectigely and my kids can circumvent it”. So what you should just be saying is that you don’t support the law.

You can be saying you support it on the condition that it’s implemented poorly enough that it should be circumventable. That just doesn’t make sense.