Well, they probably wouldn't have gone into Dark Island out of respect for Susan since she would definitely have voted no. I don't remember Edmund being badly behaved in the story except at Deathwater Island and Caspian and arguably Lucy were behaving badly there too. The bad behavior had to do with the prospect of limitless gold rather than particular characters. I imagine Peter and Susan, had they been there, would have been acting the same way.

@col-klink maybe it's referring to the movie. 

I think it would have changed the whole balance of the story — much of the focus is on Eustace as the "new" character, and if there were four familiar child characters to play him off against, instead of only two, then either there'd be less room for Eustace's character development, or else the four individual Pevensies would play even less of a role than the two of them do in the story as it is, and it'd feel like they were there to "make up the numbers" more than to add anything to the plot. Or the story would possibly become a lot longer and more complicated in order to give Peter and Susan some adventures of their own.

Even as it is, Edmund has only a very minor role in the story — his only really significant scene is where he has the conversation with Eustace after the latter's undragoning. There's no episode where Edmund is centre stage, unlike Lucy, who has an adventure all to herself in the Magician's house, as well as her being the main point-of-view character for most of the story, other than in the parts that focus on Eustace. Adding Peter and Susan to the mix would most likely mean that either they too didn't get much "air time", or else the plot would go off in all kinds of other directions. And of course, if Eustace behaved better the whole time because his older cousins were watching him, then he wouldn't have the same character arc of awful behaviour leading to comeuppances and reformation...  

Actually I would have been rather disappointed if I were Peter or Susan in not taking the Dawn Treader voyage. Peter might have liked sea voyages and adventure more since Susan may have been more interested in frivolous things as she was at the end of The Last Battle. I think it was more of a sad experience for Peter since he was more adventurous. Perhaps Susan was just interested in being nice to people since she was known as the gentle queen of Narnia, which is something good. But of course we don’t know for certain since it isn’t mentioned in the books what both of them thought of Edmund and Lucy taking the trip.  🙂

Posted by: @coracle

@col-klink maybe it's referring to the movie. 

This could be a faulty memory but I don't remember Edmund being particularly bad in the movie either. I guess I would say in both the book and the movie, Edmund is the least tolerant of Eustace, but that's understandable since he has to room with him.  

Posted by: @narnian78

Actually I would have been rather disappointed if I were Peter or Susan in not taking the Dawn Treader voyage. Peter might have liked sea voyages and adventure more since Susan may have been more interested in frivolous things as she was at the end of The Last Battle. I think it was more of a sad experience for Peter since he was more adventurous.

Interesting take. I never thought of that. I guess Peter was pretty adventurous in The Lion, the Witch and the Wardrobe.

@col-klink Movie Edmund has disagreements with Caspian, gets his torch but not Peter's sword (so he cleans off the encrusted one), is frustrated that he isn't stepping into Peter's shoes. He and Lucy get introduced as High King and Queen, which sets him up for failure.

@coracle From what I remember, at the beginning of the movie, Caspian offers Edmund Peter's sword but Edmund declines. He only complains about it at the pool on the Deathwater Island.

To bring this back to the topic, I actually think Peter and Caspian have pretty much the same personality with maybe Caspian being a little less aggressive. Maybe that's why C. S. Lewis didn't want them in the same story more than once. It's better when you can get characters with lots of different personalities to balance things out. Of course, then you have to justify people with different personalities hanging out together hence C. S. Lewis writing about siblings in The Lion, the Witch and the Wardrobe. 

@col-klink Caspian takes out the sword: Edmund looks hopeful but Caspian only offers to let Edmund hold it, as if he were a child.

VDT offers reasons for Peter & Susan not being on the Dawn Treader, such as exams in Peter's case, and in Susan's case, she, too, had finished her basic education, when she went with her parents to USA, when her father was doing some sort of lecture tour, there. When Susan had already been acting so grown up that Mr & Mrs Pevensie thought it worthwhile to take her with them, I think she would have remained that way if she had been on the Dawn Treader, which Edmund & Lucy would find even more irritating than putting up with Eustace's antics. 

@waggawerewolf27 true. And a practical reason was that Lewis had already published Prince Caspian, in which Aslan said they couldn't come back! So he wrote VDT just for two Pevensies and a Scrubb.

I think Lucy and Edmund would have told Peter and Susan everything that happened on the voyage. And since Peter was a high king and Susan was a high queen they would probably have travelled to at least some of the same places where the Dawn Treader’s crew visited.  They might have had different experiences and other adventures.  Of course many of the places on the ship’s voyage were previously unexplored by people in Narnia, which is much like the pioneers of our own world’s history. 

@narnian78 Susan was crowned Queen.

Only Peter was High King. 

The scriptwriters for VDT were wrong in having Edmund and  Lucy introduced to the crew as High King and High Queen. They were just King and Queen, two of the original four Kings and Queens, of whom Peter was supreme.

@coracle 

I live in a country where there is no royalty so the difference between a high king and an ordinary king or queen is not so clear to me. We have only presidents here in the U.S. and they are basically the same.  There isn’t any hierarchy of presidents. Thanks for informing me of the difference. I was simply making the point that if they are kings or queens in Narnia they probably would have traveled a lot and often to other countries. The world surrounding Narnia didn’t seem as big as the Earth, but still there were plenty of interesting places to visit.  And that would appeal very much to Peter or any other king who loved adventure.

@narnian78 Sorry, I know you were just making that point.  

In VDT Lucy does talk about their previous visits to the Lone Islands, in the Splendour Hyaline, very likely including all four of them.

UK hasn't used High King at all, as far as google and I can tell. Unfortunately there wasn't a check on the script for such things - or others (groan).