Bunk beds are an interesting way to maximize space and some kids think they're very cool. If you go with bunk beds, though, it becomes even more important to establish those "private" zones, so kids feel like they've got something of their own.
Loft beds can be used to great advantage in a bunk room setting. These whimsical loft beds not only create extra sleeping space, but help to further the theme of the room.
More space-saving bedroom ideas:
Inspiring Murphy Beds
Bunking Up
Get the Most Out of Your Space With a Daybed
interesting configuration of the bunks. More clean lined than we would want, but the supports are neat. They are a little dinky, but the overall idea works.