Giants is a good place to start; MDK series are good too. Those games were released back in my earlier years as a teen, and I enjoyed them a lot. I think they'd be good for your kid to try out.

Many GOG releases have an ESRB or PEGI rating listed.

I know that. Of course many of them have ratings, if not all. But, as stated, ratings only help determine what games are unsuitable for kids in terms of violence, bad language etc. They don't tell you whether or not the game is going to be fun for kids, or whether it's going to be easy enough for them.
For instance, a full on micromanagement sim game is most likely to be rated 3+. What's the chances of a 3 year old mastering that, or finding it fun? If you're actually buying for kids than ratings don't tell you much at all by themselves. Sometimes you have no way of knowing how accessible a game is until you're playing it.

The problem with game ratings is that the only thing they take into account, is the content of the game, meaning the level of violence, any erotic imagery, etc. It says nothing whatsoever about the suitability, difficulty-wise, of a game for a particular age group. There are tons of games that are rated 3+, but which require excellent motor skills, planning skills and above all, English reading skills.
Ratings won't help you answer the question "is it possible for my child to enjoy this game". At least not if your child is under 10 years of age. Usually more if English is not your native language.

I can re-post the list of GOG games rated E and T if anyone wants to see it. I deleted the original post because of the size. Not many for kids actually.

I saw the list, actually. I haven't been on the look out for kids games, admittedly, but they aren't something I've noticed a lot of here. Those that are aimed more at kids have already been mentioned.

Maybe GOG needs an informal rating built on a user vote style system. Throw up categories like recommended minimum age, level of violence/Gore, adult content, language etc and we can vote to create a community aggregate score for each category.
An example for Fallout:
Recommended Min Age: 5, 10, 12, 15, 18
Violence/Gore: None, Comic/Light, Medium, Strong
Adult content: None, Light, Medium, Strong
Language: None, Mild Crap, Medium Shit, Fucking Strong!
A quick vote when you rate a game would be all that'd be needed and could give prospective buyers a rough guide to the content of the gameWhat? How old were you when you started playing games? Games of the 80s had punishingly stupid difficulty curves because they were based on an arcade model designed specifically to string you along just enough so you'll keep spending money BUT designed to kill you frequently enough that you had to spend a months worth of pocket money just to get to level 2. Sure there were freaks who could beat a bullet curtain shooter on one coin but thats because they spent enough playing the game to buy the machine 3 times
Damnit, now I fancy playing 1942 again...

True. But you had to be a die hard* gamer back then to even be a gamer at all. Most gamer kids now wouldn't even touch games if they weren't more accessible than a disability convention.
It's a much, much, much wider demographic than it used to be and subsequently includes no end of kids who have to be doped up on Ritalin to even be able to string a coherent sentence together. That and kids who are naturally just stupid.
* Avoiding the use of the word hardcore.

Yes, but there are at least two different kinds of difficulty. There's a complexity level and a skill level. All the games we remember fondly from the 80s (like 1942) had very low complexity levels and very high skill levels. Anyone could get into them, but you needed to practice a lot to get far. Then compare that with a game like, say Dungeon Keeper. The level of complexity of the game itself is vastly greater than that of those old arcade games. Not anyone of any age can get into it. That's just one example, I'm sure you can think of many more yourself.

I would like to see the Mia and Freddie the Fish genre of games here on GOG. I believe the problem with getting this type of game is they still sell well and at $20 + prices even when they are several years old.

Yes, that is what I'm saying. But your reply to Navagon indicated that you thought no kids of any age would have any problems getting into any game. I was trying to show you that they would, and your reply to me seems to indicate that you agree. So which is it?

Many kids are way tougher than people give them credit for, failing at something because its hard can motivate them to actually put in some effort and learn how to not suck. My point was that if something is hard, they need to be taught how to do it better, not make it easier for them by reducing difficulty or complexity. I was playing flight sims when I was 8 and sucking really badly at them, I did the unthinkable and RTFM, I learned about drag, stalling and deflection shooting and then started being good at them
Other kids of course are just crybaby sissies

Oh man, I'm suddenly reminded of this.

good idea