It doesn't block content, but X3Watch monitors what websites you visit. If you use that in conjunction with DroidWall as suggested by Sparx, or use an App Protector to block/limit other apps, that may help.
Update: Covenant Eyes now has an app now available in the Market. As with X3Watch, it doesn't block sites (though it may in the future) but reports what websites you visit. It is officially a beta version, but seems to work well. The app itself is free, but usage requires a Covenant Eyes account - which also covers usage of any programs on the PC or Mac.