KidSafe Security Key - http://www.thinkgeek.com/gadgets/security/936f/, one option.
Try to raise the kid right. (And I'm not referring to you specifically, Scully, but all of us parents.) If you think you have reason to believe something is going on that shouldn't be, become technically savvy enough to monitor their computer use. If you can't, restrict their access. Don't let them have a PC in their bedrooms. If they prove to be untrustworthy, shut off access completely.
My son is six. He's not old enough to be that technically savvy, but he still has a locked down profile on the PC, which is in the living room. He can't do much of anything on it but run the applications he has icons for on the desktop, even if he knew how.
With Internet access, start from a state of no access at all, then allow them access to the sites you want to let them have access to. Check logs on occasion to make sure they haven't bypassed the security. If you even suspect that they have, lock it down completely.
Hopefully, we raise our kids to be more trustworthy than this, but...