While I can see your hesitations, I think the solution is rather straight forward: block the Google Account settings behind parental controls. I don't think you'll want your kids logging out of their parental controlled accounts so they can create new ones anyway, so that's probably a good idea regardless of the we webview they can trick into opening Google.com.

You'll find webviews inside most apps because your average weather app developer isn't really interested in preventing kids from using their privacy policy webview to access porn.

I don't get it myself (why not just launch the default browser instead of adding a webview?) but I hope you'll see that these types of workarounds are not unique to Google's settings.