iOS is better for them, GrapheneOS isn’t going to / can’t do what they want, it’s not what it’s designed for, it’s literally designed to do the total opposite of that.
The closest they’d get is using DNS to filter out inappropriate web content, using the sandboxed play services on a child acct so certain apps are restricted, and having Family Link on their phones for the approvals of things. But it’s not going to be as integrated as what iOS and Google does. Goes way beyond generic content filtering, screen time, customizable shutdowns around homework time and over night hours where kids wake up and hop on devices etc.