Among the OSR blogarati, such a thing is "rules lite" and weighs in at 36 pages; it's bought and the purchasers are never heard from again (perhaps "everbrown" is a more apt name for such a product from a marketing point of view). Among the rest of the RPG universe, it's a way to lure new people into more expensive (and thus lucrative) games; the longed-for gateway drug that will finally steal the audience away from WoW. I'm not sure such a thing is even possible, but let's explore the possibilities.
I believe there are a few criteria that a "family RPG" needs to meet:
- It's got to be accessible. Weird settings that require a lot of acclimation and understanding are going to put off the average family, which should be looked at as a "quadruple casual" gamers, if that. Go with something at least vaguely familiar; a wildly popular IP is a choice, but it runs the risk of alienating folks who just don't like Harry Potter, Tolkein, or Iron Man.
- It's got to be easy. Easy does not necessarily equate to "terse" or "short". A set of rules that's four pages long and packed densely with information, jargon, and acronyms is going to fail. Reading level should be no more than 5th grade.
- Corollary to the first criterion: It has to be quick to start. Monopoly takes about as much time to set up as it takes to count out the money. Risk takes a little longer, but not much. Spending an hour on character creation is a non-starter.
- It's got to be geared for 3-5 players. That means 3-5 classes, or races, or roles, or whatever. No more, no less. Save the rest for expansions for the hard-core players. You need to have one for mom, dad, and 1-2 kids, and deciding should be as easy as choosing between the car and the top hat. Why do you think "Hungry Hungry Hippos" is made for 4 players, and Monopoly works best with 4-5?
- It's got to be forgiving. You trade two railroads for Baltic Avenue, you won't automatically lose the game, even though it might be harder. Same thing if you roll four 2's and a 1 when defending Kamchatka. The sort of "you are punished both for stupidity and for bad luck" that many old school games embrace just won't fly.
- It's got to be fairly winnable. It could be a team win, or an individual win, but it cannot be a situation where one player serves as the referee/foil for the rest. In a family scenario, nobody wants to be felt "ganged up on", and that's what the DM is, basically, in traditional D&D-type games. Normally, that's counterpoised by the near-omnipotence of the DM, which in turn is balanced by the fact that people will leave the group and find a new DM if he is consistently unfair. However, plug that into a family scenario where mom had a bad day at work, or Susie stole Jimmy's last Popsicle from the freezer, and it becomes a madhouse of metagaming mayhem. What's lost is the last piece of balance; in a family game, you can't just go find another group, because you're stuck with the family you've got.
- It's got to be fun. Because nobody plays a game twice if halfway through you have to ask "when does this become fun?"