Requiring payment for access would also work. Maybe $10 for a lifetime membership, otherwise its read-only.
Children can only participate with these companies because the product is 'free'. Same with p*rn and free2play games: eliminate the 'free' component to effectively block them from children (at least without parental consent).
Not too hard to get around. I have friends who used to buy American Express gift cards to get around having to pay online with a credit card so kids could easily get around any payment restrictions. I think all a paywall would do is make life more inconvenient for the average user.
Children can only participate with these companies because the product is 'free'. Same with p*rn and free2play games: eliminate the 'free' component to effectively block them from children (at least without parental consent).