Your analogy needs modifying. If we remove the laws, a child could walk into the store and buy beer with their pocket money, no questions asked. This isn't the same as them browsing the internet no questions asked.

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.