Their argument is entirely based on the assumption that the child can change their DOB on the device at any time. That's trivially easy to avoid with a simple admin password requirement. If this was implemented in any competent way (granted, that is a lot to expect of legislators) the DOB would not be able to be changed once the device/user account is setup, or would require an administrator password which obviously shouldn't be given to the child.
But they turn around and say this is good and how things should work:
This app lets you chat with people on the internet.
If you're a kid: ask an adult before chatting online.
Yeah, the kid that's willing to change their device settings is definitely going to go check in with Mom before they access something they know they shouldn't be on. That's just an unbelievably bad argument.