It’s worth making whatever kind of submission you can manage for this. The page even specifically notes: “Due to the short timeframe of this inquiry, the committee would appreciate submissions being limited to 1-2 pages.”
This bill will affect all users of social media in Australia, and may have a particularly negative effect on Fediverse platforms (like Lemmy and Mastodon) which are operated by individuals or small organisations that may not have the resources to implement age verification.
It should be enough that if you’re not kid friendly say so plainly and don’t make it easy for them to get in / stay in if they get caught. If you as a parent don’t take any initiative from there to keep your kids away from those places that’s on you.