Jasonofthenorth: Or, if you don't have kids, what do you recommend anyway?
Max: The Curse of Brotherhood is a nice kid-friendly puzzle platformer, and currently discounted on Steam (until dec 1st). Not sure if it's good for kids as young as yours though, as there is one big scary monster you sometimes have to run from, and while it's not about fighting and killing, occasionally you have to dispose of mean goblins by using your environment and creating traps (IIRC they'll vanish in a cloud of smoke when defeated).
Fight the Dragon might work as a kids game, too, on easier difficulties, and you can build your own levels with blocks, but it is about fighting (and killing), so probably not what you're looking for. Same goes for
and [url=http://www.gog.com/game/oceanhorn_monster_of_uncharted_seas]Oceanhorn here on GOG.
Lili: Child of Geos is a short and simple exploration game, it does have fights but IIRC no killing or dying, and the fights are comparatively harmless (you sit on top of funny monsters and quickly have to pull flowers from their head).
Also check out
Botanicula, it's non-violent and like an interactive children's book. You click on things on the screen and watch what happens. It does contain some dark themes and scary creatures (e.g. spider-like dark bugs that suck the life out of everything they touch and let plants wither), so if your kids are easily scared, maybe that could be an issue, but check out the trailer and reviews and see for yourself.
World of Goo and
are kid-friendly puzzle games (which doesn't necessarily mean they're that easy to beat though). [url=http://www.gog.com/game/tri]TRI is also great, but possibly a little too difficult? You have to create your own stairs by drawing triangles.
Snapshot is a cute and harmless puzzle platformer, but maybe a bit boring, depending on how patient your children are (not much action).
Never Alone is another kid-friendly platformer reminding of a children's book, and you can play it co-op (as girl and fox). It involves some serious themes and somewhat creepy creatures though.
Tiny Bang Story is a relaxing hidden object game, it might work if your children like "wimmelbooks" (not sure if you have those and what they're called, might be a German thing?). So you basically search the screen for small objects and click on stuff to make things happen.
And then there are casual games like Peggle (
Peggle Extreme is free on Steam).
I think all of the above would be perfectly fine for primary school children, not sure about preschool children though.