I have been involved with a lot of similar programs. Indeed, the majority of my early gaming was in programs like this.
You'll probably have people of a lot of different ability with RPGs involved. You want to encourage these kids, subtly, to assist the other kids. (Perhaps encouraging them to play warlords so they can put their tactical ability to use.) You also want to foster a team environment, either through some kind of common cause or threat.
You want to watch out for cliques of experienced players who take advantage of other players in order to build up their own characters. If this becomes a problem, you might want to privately communicate to these players that, because of their ability, they will be held to a higher standard at the table than other players. (You might want to soften the blow by saying that you want to make things more challenging for them.)
Don't try to break up cliques in the group, as long as it doesn't hurt the overall dynamic of the group. I suspect that 4E PC interactions will help interactions all over the table.
Since you won't necessarily get the same players week after week, try to make things as episodic as possible. Try to indicate that there are other things going on in the lives of the PCs, so that they are off doing something when the player isn't around.
Don't mind a little silliness, since most of the kids will enjoy that.
Each table develops its own, as GA Fine calls it, idioculture. That is, there will be a shared language, mythology, and history that develops around the table pretty quickly. Try to build on that.
Get lots of pencils, paper, and dice for the kids to use. I recommend printed/photocopied character sheets, since it serves as a focus. I also recommend that you get a map of the campaign area and put it up on the wall. You want visual cues around you for the kids to focus on that keep them thinking about the game. Pictures of monsters and NPCs wouldn't hurt either.
If you have some kind of ongoing mystery or plot thread, be ready to alter it based on what the players think is happening. They may misremember plot elements and then make assumptions based on that, so be ready to go with it. (Just make sure that there's no big dispute going on about what might have happened. Try to stick to the truth when there is a mistake and a disagreement, but not in a way that makes anyone lose face.)
Also, be lethal. Seriously, these kids have got to learn that the characters can die. Be upfront that these characters are risking death and try to kill at least two in the first session (if you can, try to avoid killing the PCs of players who you think will be inordinately upset). You have to show the players a good example of PC death so that you won't risk real emotional outbursts later. I've found that in games like this, there are a lot of PC deaths, so get them used to it early. After the first session, just play normally and PCs will probably die with a nice regularity.
