The original trilogy is far superior to the prequel trilogy, so I can't imagine why anyone would want all those classic moments ruined by the mediocre prequels.
As far as which is more accessible/better... I think there's a strong possibility children will be more entertained by the prequels. When I was growing up, Return of the Jedi was my favorite Star Wars movie. Now, it's quite evident to me that while it's passable, it's mostly riding on the strength of it's predecessors; it's much more poorly-acted and phoned in than either the first or second film, and while it has some great action set pieces, it never reaches the heights of either film either as a popcorn movie or drama (though I do think the Luke/Vader/Emperor scenes are some of the finest in the series).
All of that said, Return of the Jedi is now my least favorite of the originals, and The Empire Strikes Back, which I once found boring, has become my favorite. What you need to keep in mind is not what will entertain the kids now, but the images they will carry with them afterward.
I can come up with other examples of newer movies that don't quite match up to their predecessors. I don't think it's good to introduce anyone to King Kong with Peter Jackson's overwrought, noisy version. I saw it before seeing the far superior original, and while the quality of the original wasn't affected for me, I think I would have enjoyed it more if I hadn't seen Jackson's version first. Likewise, Prometheus, which actually is a prequel, is a poor introduction to the Alien universe. Either Alien or Aliens would be far better. On the other hand, I think John Carpenter's The Thing and the remake of The Fly are superior to the originals, and also have their own identity, so they're great places to start.
The prequels are so tonally inconsistent with the originals, and so poor in quality by comparison, that perhaps it will have no effect. But I can't imagine Darth Vader having nearly as much gravity as a villain if his first appearance in the original film is prefaced with Hayden Christensen's incessant whining--nor can I imagine the tragedy of his fall to the dark side having nearly as much of an effect if we know how horrible Anakin was all along.
Nah. There are two main things that make Star Wars great: the thrills (which are tops throughout the original trilogy, with well-choreographed fights and space battles) and drama (again, great even in the flawed Return of the Jedi, with one thing logically leading to another as the intriguing family drama unfolds). The prequels are so confusing and chaotic by comparison that even I, who was familiar with the original trilogy for six years before seeing Episode I, found a marathon of all six movies mind-numbingly difficult.
This isn't Power Rangers you're talking about--it's a cultural milestone. Let them get the proper introduction to the Star Wars universe, then they can find whatever childlike delight they want in the prequels.
I'd prefer them to see 1 2 3 4 5 6 in that order. Though the original trilogy apparently is the better one, I too think that the prequel trilogy is much more appropriate for this generation of movie watchers.
Out of curiosity, why? By extension, I should prefer Peter Jackson's King Kong to the original, or Prometheus to the original Alien (which was released a full seven years before I was born). But I don't. I prefer the classics not just because they're classics, but because they seem much more well-made. Likewise, Star Wars: Episode I was released while I was still a kid (I was only 13), but my first reaction to it was that it was terrible. It wasn't for my generation, and I'm not sure it's really for any generation.