1. Verify the ID without storing it in your system. Someone just looks at it.
2. Visually confirm that the photo on the ID matches the person entering the building.
Neither of these apply online.
Has everyone forgotten how kids operate? They’re not clueless. They’re going to realize that they don’t need to submit their ID. They just need to submit someone’s ID.
At first they’ll just use fake ID generators and submit those photos.
If that loophole gets closed somehow, a market will appear for buying ID verified accounts for trivial prices. People will create ID verified accounts and sell them cheap for side money. The only way around this is to start storing ID information for every account to make sure IDs aren’t used multiple times.
It’s one giant slippery slope of consequences for the adults forced to submit IDs, while the people who want to work around it do so trivially.
The child is not paying for their devices or internet access. Their parents are paying and providing the needed equipment. In a way, it's like giving keys to after hours access to the local mall, where all kinds of stores can be browsed including adult magazine stores, without any shopkeeper to apply the laws.
So one solution is don't give kids the keys. Or, since their online activity leaves a digital trail, even if they did have keys, there's a chance to moderate their activity via seeing what they have done rather than police where they might go.
That's not an equivalent analogy. Freedom of movement, to stand outside a store, is a human right. It is not the same as "freedom to lurk around online spaces on Mummy's laptop and Daddy's internet account."