This is the first, simplest multiplayer arena: http://codecombat.com/play/ladder/dueling-grounds
They may also be able to play this one: http://codecombat.com/play/ladder/cavern-survival
Each kid can click “Play Humans” or “Play Ogres” (you would probably want half and half), then beat the AI, and submit their code for ranking to the main ladder. Now, they will be playing against everyone else on the ladder, but they may also get some matches against each other. If they are Facebook or G+ friends and sign into those networks, they can see each other and challenge each other to matches.
Later we’ll hopefully expand this to make it much more natural to play with one’s clanmates, but that part isn’t ready yet.