As a summer camp counselor, I've had to introduce many many games to kids in the age group you are talking about. If you can't get everyone involved in playing, (which you often can't in a game like this) here's what you can do.
Demonstrations!
As with most games, D&D 4e is most easily understood by actually playing the game. To that end, we want a quick and easy way to get the players into the game actually, you know, playing.
To that end, you're going to want to create the characters yourself before hand. Make a 1st level party- probably the Fighter, Wizard, Cleric, Rogue iconic one, but any party with all four roles filled out should work. This is so that the players don't have that bar to entry of "making a character" that takes up a lot of time and can sometimes lock new players into analysis paralysis. Make sure you have all the power cards handy, and can hand them over to the players who end up playing them.
I assume you're going to have more than 4 people interested, which is great! Explain that this game is something a little different than games they've played before- that it's like an open-world video game, but with dice and people, instead of a computer. Then see if you can get four participants who are excited about that prospect. Give them a little overview of your four characters "this fighter is the toughest, and is intimidating, and loves to drink beer!" (maybe not if you don't think that's age-appropriate). "This wizard is a master of magic and is super-smart!" etc, etc. See who is into what, and let them fall into the roles they desire. You're ready to play!
If you feel like prepping your own adventure for this, great! Try to make it a little more on the simpler side, with some opportunity for role-playing, but focusing more on the "gamier" aspects. These are easier to grok for most first timers who aren't used to role playing games, and as such will more than likely come in with different expectations.
If you don't feel like making your own scenario (which is a completely valid way of going about it) I would suggest running Keep on the Shadowfell as an adventure for your newly minted party.
Let others spectate, and be patient in guiding the players toward their goals. Explain to them off the bat that they can attempt whatever they'd like, and whenever they try something, they have to roll this 20 sided die. The higher they roll, the more likely that they will succeed! Whenever they ask to do something that requires a roll, be sure to explain what that roll is. "Okay, you're trying to climb up the side of this building, that is an Athletics check. Roll and add your Athletics score to the results." Eventually, the players will start to know what to roll for and how to add it up.
The biggest hurtle after this is combat. Combat is crunchy in 4e, but should go smoothly if you have the patience to let them go a little slowly. Explain how movement happens in squares, and each turn they can perform one Standard Action, one Move Action, and one Minor action. Don't worry about downgrading actions until someone asks about it. Let them know that their powers will tell them what type of action it is. Let them take their turns, correcting any mistakes along the way with a gentle hand. You'll be surprised by how quickly they start to pick it up.
The Don'ts
Make sure to keep them moving. The thing that will kill this game is letting the players goof around and not go anywhere. Kindly and subtly guide them toward the next part of the objective. (Another reason I suggest Keep on Shadowfell- it starts with a combat encounter.)
Don't get too bogged down in corner cases. This stems from the first warning. They don't need to know all the specific rules and interactions. Just the general idea of "try a thing, roll a die, see if you did the thing." That and some roleplay stuff where they talk to people in the town, and you've really sold what this game is about. Good job!
If you have the time, maybe do 2 sessions, switching out who the players get to be in between. (You could even let some spectators jump in and control the characters that are being played already.)
Then, you blow their minds at the end with "And in this game, you can make your OWN characters and story, not just follow the ones written down by someone else."
