Start your security thinking at an earlier point than anti-virus: Create each user an account without administrative privileges plus an administrator's account with a password. Never use the admin account unless there's no other choice! That means, a normal user is allowed to download anything, but the installation has to be done "as another user", meaning the admin with the password. Needless to say your boy shouldn't know it!
As for which anti-virus is good or not, there's actually no fool proof one! Many of them also have got stuffed with features that have nothing to do with viruses, like "Internet cost saver", defragment, temporary files cleaner, updater for other programs, backup reminder etc. Skuzzy has often recommended to stay away from Norton and McAfee for good reasons, so you can count them out at once. Eset seems to have the smallest negative effect on games. From the freebies I prefer Avast, but with a grain of salt: The normal install package is somewhat bloated, but the custom install will let you choose only what you think you really need. MSE is light but AFAIK it wants Windows' automatic updates to be on, causing unwanted Internet action amidst the tightest furball at the worst. For AVG I've created somewhat of an allergy, even the paid version wants you to install all sorts of additional features. I've seen their free PC TuneUp cripple totally valid programs by removing parts of them as junk. Avira has a good reputation of finding viruses some others don't. It has a slightly home coded overall looks, making it somewhat clumsy to use. Whichever you choose, remember that everything man-made can be reverse engineered, bypassed, cracked...You're not safe, but you can build a safety net to minimize any potential damage.
