HotWyre: I was thinking more of a authentification code lockout on the play button. that to me seems like the most direct way to
lock out a child from playing a game to violent for them. Obviously if the child is very good with computers they can get around anything, but, I would imagine that percentage of kids is low. It's more of a convience thing for the parent so that they may prevent their kid from playing something they don't want them to. As for the ratings, it's an idea that reviewers could fix with their comments. Put a few age brackets to pick from or choice descriptors that parents can breeze over to see what sort of content a game has in it without having to buy it. I think anyone who reviews a game can tell people what to expect. That could be a rating system.
HypersomniacLive: It sounds to me like a situation where the child is allowed to use the PC account of the parent(s) where all sorts of games may be installed. I think that an authentification code lockout on the play button is overcomplicating things when there's a much simpler solution - the parent creates a limited account on the PC for the child(ren), and installs only the games they see fit for them to play. This is the most direct way to lock out a child from playing games that aren't deemed appropriate for them by their parents.
Out of curiosity - is there any digital store that offers the sort of lockout you suggested?
Regarding the ratings system, I'm pretty sure PEGI ratings are listed at the bottom of most game pages, which, as far as I know, give the information you're looking for without having to buy the games, so I'm not sure why you favour the ESRB rating system over what's already in place.
I thought GOG used to show PEGI/ESRB icons on game pages, but I haven't seen any in ages. A brief scan of the top listed sale items (all modern AAA/indie games) are conspicuous in their lack of any ratings.