Some good resources mentioned in the posts. It is however important to think beyond running filters, and remind children (and us) about digital citizenship and how our online postings become our body of work, ie, online resume. We have to go beyond only avoiding bad behavior, and move to promoting good behavior and postings. Good digital citizenship involves posting good, useful material online, encouraging others, etc. I know someone very active in the technical side of education in Ohio, and when you look at his online postings, talks, etc, he is incredibly productive in the tech arena.
Working in a school, I have seen first hand the damage social media postings can cause, and sadly, it often comes from parents going on a rant without accurate information about an issue. Also, our teens need to understand the level to which employers will go to search online postings of a potential hire, and how online postings can hurt their chances for a job.
In short, filtering is good, but only the first step.