Here's another tip -- a lot of wireless routers (and regular routers, I assume) come with the option to block harmful sites. A parent can easily configure those routers to block any sites that aren't child friendly and set up other filters.
A little research is in order here, but most companies that offer parental controls make a big deal out of it in their marketing. It's worth mentioning that the filters I've seen are dynamic -- a company such as Norton (for example) will update a database of harmful or are not child friendly and the router will utilize that updated list when screen Internet addresses.
Considering the amount of garbage floating around on the Internet and the fact that kids tend to not listen to warnings from parents, a parental filter is a feature that is well worth considering.