In case you missed it, it's Teach your Kid to Game week, an idea that I 100% approve of, even if the idea of an entire week to it seems a bit unnecessary.
However, on its intended purpose, I can lavish nothing but praise. I've had the opportunity to run games for young kids before and, let me just say, that it was highly rewarding and actually quite fun. Very imaginative children will think of solutions to problems that no rational adult could ever conceive, whether or not said solution is physically possible.
There are plenty of tips and advice out there for running games with kids, but let me just reiterate a couple of points I feel need mentioning as often as possible.
#1: Don't play for hours. Kids get tired, have shorter attention spans, and basically will burn out faster than you will. Limit the game time to two hours at the max, maybe even cut it short to an hour-and-a-half. You can get in a little adventuring and try to end it on a cliffhanger so the kids actually want to play again.
#2: Tone the violence down. I can't stress this enough. If you want to keep the younger set comfortable, you'd better run a level of violence appropriate to a Saturday morning cartoon. It seems like an obvious thing to give as a piece of advice, but I've seen some GMs who don't consider killing to be too adult a theme for young kids. Sure, if you're playing with teenagers, start with the chopping, but if you're running a game for younger kids, say in the pre-teen range, you've got to tone it down. Knock out the bad guys, maybe hurt them a little, but don't do anything that would make the kids (or their parents) uncomfortable.
#3: Pick the right setting. Selecting a rules-lite system is easy. Selecting a setting that won't bore pre-teens to tears is a challenge. Some might like fantasy, some might not. Talk with the kids prior to the game and try to find some common ground. Light fantasy works best, especially with a touch of humor. A setting like Redwall (with its animal protagonists) or the early Harry Potter (with its lighthearted take on magic) are good ideas to mine inspiration from, as are superhero comics, especially with the boys.
As for what system to run, if you're running a game for pre-teen kids, say in the 9 to 12 range, you might not want to bust out the 4th Edition books. Try rules-lite. Free is even better because then, if the kids show an interest, they can get a copy of the rules for themselves and read it at their leisure.
I'd be likely to pick Risus (for any setting really), the Tunnels & Trolls sorta-clone Tunnelquest (for a fantasy game), maybe Microlite74 (for the slightly older set), or the old B/X Dungeons and Dragons (with plenty of house-rules to keep the kids from an untimely death). Board game half-RPGs, like the ancient Hero Quest game, are an even better idea, as most kids have experience with board games, so it won't feel as weird to them.
Some good advice can be found from Escapist writer Bill Walton and Drive Thru RPG has a bunch of kid-friendly selections available.
How about you? Have you ever run a game for kids? Maybe your nephews or nieces, or even your own children? How did it go and what advice would you share?