MitID is 2fa. You log in with username, then you have to open the app, enter password or scan biometric, then scan the QR code of the screen* and you are logged in.
He would need to be next to you every time you log in. I think that is too high friction to make it feasible on large scale.
* Assuming you open the website on the Desktop, and MitID on phone. If both on phone, skip this step.
Also, I reckon most children know the password for their parent's phone or computer, and many more will find out if there is a highly motivational factor for doing so. How many exhausted parents just toss their phone to their child to stop them whining?
I suppose it could be a biometric sign-in with facial recognition or fingerprint, but again, that's a tonne of friction for the whole web.
It's already the single sign on for government websites, banking, healthcare, digital post, insurance, law (sign contracts) etc.
Shit man, you can get divorced through that. I really hope most parents don't give their kids access to it.
Or you can just text him a screenshot of the QR code. You could probably even automate this.
~Maybe~ you can video call, but again it's adding so much friction. Nothing is 100% secure.
The only high friction part of this is that someone needs to write the software for it, but that doesn't seem like all that difficult of a project and open source solutions are likely to appear within weeks of social media requiring it. If there really is no information shared with the other party beyond "yup, user is over the age of maturity" you could even run this as a free public TOR service without fear of ever getting caught.
High risk, low reward.
If he throttles request to stay under a threshold, if the agency knows about it service they could use it and see which account does age requests at the same time.