I think it greatly depends on the type of music they want to play but there is a good thread going
Starting a band and need advice.
It talks a lot about the workings of a band and what role each person would fill. I can't help but think though that getting them all together and just seeing what happens would be really interesting. Since they are younger I have really no idea what kind of music would be appropriate for them to practice with.
Easy To Play Songs?
That is a link for easy songs (at least on guitar) that you might want to take a look at. I'm sure there has to be one or two that would be appropriate. Let us know how it goes!