It's a huge effort to create a google account for a child without gmail. I had to sign my kids up using a fake age and a selfhosted email account. I could fix the age afterwards.

I'd estimate that 99% of under 18s on google have GMail already.

This whole thing is just a cover to read everyone's mail anyway. It's bizarre that it's illegal to read someone else's physical mail in Australia, but for email it will be illegal not to.