I'm fixing up my dad's old work laptop (which runs winXP) as a gaming computer for my little siblings (5, 6, and 9 years old). My goal is to move them away from the run-and-gun/zombie slaughterhouse games that I and my brother have inadvertently introduced to and instead introduce them to some productive, possibly (but not necessarily) educational games. What games would be good for this?

Right now, my list is quite small:

Lego Island (a favorite of mine as a kid)
Oregon Trail
Driftmoon (?)
Frogatto and Friends
Woodle Tree

P&C games or anything with a simple-to-use control/interface is preferable.

Edit: Just realized maybe I should clarify that I don't mean exclusively on GOG.

Oh, I forgot to mention I'm not just looking at GOG. Thanks for your suggestions though!

You're quite welcome. Personally, my parents made sure to make the barrier between video games and reality quite clear, seeing as other parents were idiots having to deal with their kids punching and kicking because they saw it on Power Rangers. In retrospect, I thank them.

The Lego * games (Lego star wars, Lego Batman and so on). They are actually surprisingly fun!

For the 9 year old, Monkey Island, Sam & Max, The Dig, Syberia, Broken Sword and other classic point & click adventure games, plus management games like Sim City, Theme park, Theme Hospital and so on will probably work well.

For the slightly younger ones, I don't have quite as much experience with games for kids in that age. When I was that young, I had an NES, and had no issues with playing platformers of that era, so games like Super Frog (don't get the HD version...), Jazz Jackrabbit, Oozi: Earth Adventure and similar games will probably work.
Also, if they are learning how to write, then why not see how they react to something like Scribblenauts?

Definitely Night of the Rabbit! It's easy enough for a younger set but challenging too....;-)

Stacking, Costume Quest.... and if you're willing to buy the bundle on Steam, Cosmic Osmo was the first game I introduced my 2 year old son to...by kindergarten he was fixing Mac's for his teachers ;-p Oh and that includes Spelunx and the Caves of Mr. Seudo....another amazing classic that kids and their adults find amazing and humorous.

Both Edna & Harvey games are fun.

Crazy Machines are amazing in terms of teaching logic - it's too bad GOG doesn't have the Logical Journey of the Zoombini's cause that was an amazing logic game for kids - and well adults too.

Machinarium, Monster Loves You (perfect for younger kids that want to try different moral options), Scribblenauts, The Tiny Bang Story, and The Book of Unwritten Tales....

Oh, who can forget about The Learning Company classics like Reader Rabbit and the Super Solver Series?

What about Minecraft? It doesn't have many defined goals so mostly revolves around creativity and problem-solving.

You'd have to watch with a hawkeye, considering how easy it is to get mods and go online.

Peggle? Great for the older audience as well. In fact pretty much all PopCap games, such as Plants vs. Zombies.

Actually, Minecraft has been really helpful with my little brother's OCD. I just load up an empty world on creative mode, and he builds villages.

Is this what you meant!

www.oneonlinegames.com/download-games/windows-xp-games-free-download

these are "free" downloadable games I don't know if you need an account with them or anything?

- Professor Fizzwizzle
- Professor Fizzwizzle and the Molten Mystery

- And here's a ridiculous amount of levels for the first if you decide to get it. The game comes with a level editor and these are all the levels from the official level portal forum from a long time ago.

Normally fairly cheap at $7 each, but both are $3 right now for a sale. The hard levels can get quite devious even for adults. Fun platforming brainteaser game; like Lode Runner with more options.

Doesn't Big Fish Games require games to be run through a client?

My son (6.5 Y) plays Sid Meier's Pirates (the newer one). Colorful, warm game with very limited violence.