Once I got over my initial shock at hearing about a fanboy parent who failed to indoctrinate his son with the awesomeness of Tron, I started thinking about how a kid could be so eager to see a movie that's openly been billed as a sequel, even though he didn't have the foundation of the earlier production.
Obviously it's the sheer power of the marketing by the titanic Disney machine that's Master Control-like grabbed the consciousness of anyone who likes science fiction or action movies with cool special effects, and the fan-fed hype that's followed the advertisements and strategically leaked tidbits.
But what's more interesting is that this kind of young fan break with the source material is something I've only seen and heard about before with the Star Wars franchise: kids growing up on the prequels or the Clone Wars and loving them, without having seen the original trilogy. To my knowledge, this hasn't happened anywhere else.
Granted, there aren't many other franchises that have been around long enough to have tried reinventing themselves to offer this kind of generational disconnect. Star Trek had its recent reboot, but the original Trek is so ubiquitous in our culture that it's hard to imagine that even young mainstream movie-goers heading to Abrams' ADHD-inspiring action and camera glare fest wouldn't have had at least a little exposure to The Shat and co. There was Battlestar Galactica, but the new series really wasn't something you'd want your kids watching, so I doubt it's something that's happening with this show. It's possible this sort of thing happens with anime franchises. Certainly some of them, like Gundam, or the various installments of Macross, have been, if not continuous year-over-year, then at least popping up at regular intervals to allow new generations of viewers to latch on without the benefit of the source material. But even then, regular anime viewers tend to be a fairly devoted bunch, and I'd venture to guess that any kid who's gateway to a particular series has been through one of its newer installments would probably go back and explore its origins. You don't necessarily see that with Star Wars though... some kids seem to be happy just with what's in front of them.
But with Tron... sure, you probably don't need to see the original; the trailers seem to indicate that Legacy will give enough backstory on its own, and with a new character entering the game grid, and with that other world being totally new to this new protagonist, the younger audience won't be alienated without having seen the original because they'll be able to comfortably put themselves in the new hero's shoes (or electronic boots or whatever). That being said, younger viewers should see the original Tron, not just because it's flashy and the old-style graphics are sufficiently old and different as to possibly be interestingly alien to kids who've grown up with slick near-photo-real games, but because it's a great story, one with many layers that they can ponder as they grow older. As much as I want to see Legacy, I suspect it's likely to be a simple, straight-forward (though probably enjoyable) action flick, rather than the thoughtful (and action-packed) film that the original was. Kids need to see movies that are not only cool to look at, but are smart, especially smart, as well.