I'd like to suggest adding a search by rating option or a kid/teen category to make finding good stuff for younger players a little easier. :)
I'm a mom, and a longtime gamer. I have saved a LOT of my old games (because I'm that sort of person), and so i have several games that my older son can now enjoy, but there are a large number of games on here that I'm slowly purchasing for him, as I don't have them in my library anymore, or I need to get the compatibility patched versions available here. We've already downloaded Beyond Good and Evil and Myst for him. He likes both action games and puzzle/point and click games.
My point is that there is a LOT of great stuff on here that's both challenging and not inappropriate for kids over 7, and I think more of that stuff might get sold if it was easier to search for it.

A good idea with one slight drawback: there's nothing really here for kids. Manhole is one exception, but that's about it.

Depends on the kid I guess. There's lots of games like the incredible machine and several others that aren't exactly violent, but may be too hard for the wee ones. The 9 to 12 year olds however should be more than capable of killing a some hours with them.
I think it wouldn't hurt to have such a feature, classifying games by their esrb rating.

I don't own any of the too explicit games on here (except Duke Nukem and Painkiller. Far Cry isn't that bad.) and I'm 16. My 9 year old brother plays a few of the games I own but only underneath my supervision.
Good idea though.

I would have to object to that assertion. I'm not talking about a very young child, but an older elementary student with pretty advanced problem solving skills. I can easily think of a dozen titles that I consider safe and interesting for him to play, actually:
Beyond Good and Evil is fairly gore-free, mild violence, with a good story and some puzzle elements. We have this game, and he likes playing it, and needs only some help from me. Myst is certainly appropriate for kids, even if he needs my help with the larger words in the books. All of the Myst series fits that criteria. Many of the other point-and-click adventure/puzzle games do (I do except the stronger horror titles, though). I'm considering Earthworm Jim, as well.
The Incredible Machine is also a fantastic problem-solving game.
Unless we are talking about really young kids, there are a number of good things here.

When you look at the description the E and T rated games are not really for kids

Some of the earliest games I played were the Sierra "Quest" games, although while they may be fine for kids as far as content goes, they are quite difficult.

Mostly the problem is in how you'd go about dividing them up. We don't have ESRB ratings here. So I don't know how comprehensive they are. PEGI ratings only go back as far as 2003 and then not for every title.
Even then, there's the issue of difficulty. Just because a game's violence won't scar a kid for life doesn't mean that it's easy enough for a kid to enjoy.
All in all, I don't know how GOG could implement something like that without a lot of work involved.

What if you are already a customer here, with kids, who wants to both: 1) share good games that are otherwise out of print with their children and 2) support a company you like? There's easily 200 dollars US worth in games here that my older son can play now, and in a couple of years I think he'll be ready for the sim games or the citybuilders.
There are actually games here I'd buy for him to play that aren't a priority to buy for me.
TBH, there isn't really any sort of recreational game store that IS kid targeted. If we want educational games, there are lots of those, online and for purchase, but he gets bored with those easily because with nearly all of them, the focus isn't on the gameplay or the story, but the subject being taught. They are simply education-focused casual games, with all the shortcomings of such games. I want him to enjoy games for the sake of games, too.
Maybe the way to go is something like a "gamer parents recommend these games" thread with reviews and age ranges mentioned in the listing. A community-based approach might work if the infrastructure, coupled with the age of the games doesn't make for an easy search criteria/genre sort.
In any case, i'm used to vetting everything that isn't kid-specific for my son. So he gets to watch select episodes of Dr Who, Eureka, Warehouse 13 and other series (we restrict viewing of overly horror, adult or gore-themed episodes), gets certain non-age-appropriate books read to him chapter-by-chapter, so I can skip stuff or we can discuss things with him (the Hobbit, most of the Hitchhiker's Guide series, etc), and we do the same with films. Maybe my POV is skewed because of that.

Videogame stores don't have a kid's section do they? Movie stores do though, and so do clothing stores, and music stores, maybe you're on to something.

Some advice, if you allow your kid to have a debit card in his early teens he will just buy all the things deemed inappropriate like I did. I have bought quite a few 18 rated games here my parents aren't aware about.

I had one too, and I ended up spending my college money on crap, so, no, I won't be doing that. :) He can have one when he's working.

Well, that makes perfect sense. And no, I don't believe in wrapping kids up in cotton wool or anything like that. I sounds to me like you're taking a very responsible and reasonable approach.
But because of this there can never be a categorisation that will really work for you. You're going to know better than ratings boards what's right for your children.

That sounds like by far the most reasonable suggestion, really. If you're interested in starting such a thread feel free. Ideally it needs someone who's willing to update it on a semi-regular basis though.

Well, you can turn off the gore and strippers with parental lock in Duke Nukem 3D... >_>