A classic game that can still be found is Bugdom by Pangeae. It's an old game from back in 2000, originally for the Mac, but there's a PC version if you want it.

It's harmless strategy fun, great for little kids.

Seconded on Portal 1 or 2. The puzzle component is always a plus for developing minds.

 

It mostly depends on their personal preferences. You can't just launch The Sims and hope for the best. At least not with that fourteen years old. They should know what they prefer by that age. If they like racing with some action, pick Flatout 2. You know what I mean. I bet that fourteen year old already knows what to install.

That younger kid is harder though... Maybe atomic bomberman? If it's a girl, definitely aforementioned The Sims. I don't really know, ten years old... It's probably too soon for violence.

 

I'd say Baldur's Gate, Icewind Dale, XCom, anything Bioware makes.

 

Planescape Torment for the 14 year-old.

 

Used to love playing Age of Empires at that age, and had the benefit of occasionally teaching me some history.