The posters who have said that actually monitoring the kid's internet access is best are correct, however, that's not always an option, and not what the original poster asked for.
My reccomendations:
For actively filtering content, try Kids GoGoGo (
http://www.makienterprise.com/kggg/kidsgogogo.html ). It's pretty good and reasonably priced ($30, daily database updates are $24/year), although I want to shoot the person who came up with the name.
Alternatively, and this is often a better option if there's a discipline problem, you can install monitoring software like GURL Watcher (
http://www.quicomm.com/gw_overview.html ) and manually monitor the kid's browsing. It's optional filtering ability is limited (keyword-based), but it gives the parent an opportunity to manually identify inappropriate content (which is generally more effective than electronic filters) and discuss it with the child. And, face it, even with a filter, the kid
will eventually come across some questionable material. (Cost is $19.95).
Ultimately, any setup will be a tradeoff between how effective it is and how much time and effort the parents are willing to put into it. But I would assume that a 7-year-old would not be actively looking for porn.