I think in certain areas of speech and expression, should be out of children reach. If you limit them on a parental level, you stop the child from growing up thinking the same thing. It has to be a relevant topic though, if you crush free speech you will lose out on everything we stand for.
Limiting adult freedom of expression to protect children depends on how you define limit. If we are talking about age limits on things such as adult movies and books, then yes, it is necessary. However, that doesn't limit adults - it limits children. There should be age limits on some things, but outside of that there should be freedom of expression.
Yes, I think sometimes it is very necessary to limit adult freedom of expression to protect children. I have seen adults in the world, many of them parents, who don't seem to realize that their words or actions are not acceptable in front of children. Since so many of these people are lacking in the ability to safeguard the children, then society will have to do it for them.
Much of what people say is inappropriate or unacceptable in front of children is based solely on emotion and tradition and not on a logical or analytical consideration of how it will effect the child. There may be things we say to children that hardly anybody thinks of that could be screwing them up. As a kid I heard things about the new age and sending your thoughts to the Universe and how beliefs effect reality and I had problems being neurotic about controlling my own thoughts because I thought if I had any stray negative thoughts that would cause things to go bad. My anxiety levels skyrocketed up until I decided it was all BS. This effected me for several years and may have had a bad impact on my grades. So something seemingly innocent may screw up a child because adults expect children to take words the same exact way they do.
I'd be less concerned about sex, toilet humor, or violence and more concerned about being literal and concrete when talking to children and not using vague, feelgoody sayings when the child may not understand that that's all it is.
Consider the things we want to shelter children from: profanity, nudity/sexuality, and violence.
The very concept of profanity is absurd. Words are sounds that we use to express ideas. For some reason, it is accepted that some of these sounds we can make are "bad" sounds or "dirty" sounds. If I say "damn it", that might be offensive, but "darn it" is widely accepted as ok. They mean the same thing, so it's not that the word is unkind or hurtful- "damn" is just a bad word. It's a sound. The idea that some sounds are worse than others is completely ridiculous.
Nudity and sexuality are a part of life. Everyone has a body and sexual organs. Every person is the result of someone having sex. We all emerged from our mother's womb and most of us were fed with her breasts. It's life. Acting like this is all some sort of dirty secret is far more unhealthy than explaining sexuality to a child.
Violence is more difficult. It is also a part of life and there will always be violence in the world. Once again, I believe it is more healthy to explain violence to children than it is to shelter them from it.
Even if children shouldn't be exposed to these things, limiting speech "for the children" is still a bad excuse. If a parent doesn't want their child exposed to things then it is their responsibility to keep that from happening. Sooner or later they're going to be exposed to it anyway. Children aren't as fragile as we make them out to be.