Hmm... this isn't an easy question :I
I'm torn between 1: believing- as a matter of principle- that people should be allowed to freely express themselves without fear of being silenced and 2: my own personal opinion that some things shouldn't exist and should never be seen by anyone, least of all children.
As darthmongoose said, it'd be ideal if people knew how to protect themselves from everything they might stumble across on the internet and were wise enough to take things with a grain of salt, but that isn't the case and never will be. Especially for kids.
However, if you try to impose a set of rules for what can be shown and said, you run into another problem. Who gets to decide what needs to be censored? You're giving this entity/person a ton of power in allowing them to control the message, and they very well might use it to establish a "ridiculous and discriminatory" standard for what's acceptable.
I can say with certainty that kids shouldn't be able to so easily access stuff with sexual content, graphic violence, or harmful messages. As an adult, I'd like to be given fair warning so I can avoid this content as well. Everything beyond that in terms of censorship isn't so clear-cut to me.
Edit: (And even to that, one could ask, 'But what constitutes sexual content, graphic violence, or harmful messages?' My definition for these probably isn't going to be the same as yours. So.. actually, nothing is clear-cut after all -_-)