The team is level 18 now, and leveling at a nice pace. There's a good side and a bad side to power leveling them. The good side is that I can run instances quickly and repeatedly, which means that they are highly likely to get the drops they need from any of the bosses I kill. The bad side is that I get pretty much no experience actually playing the classes. This isn't entirely bad since none of them is going to replace the shaman as my grouping/raiding main character. And once I get to a point where the group is taking a more active part in instances, I'll get something of a crash-course on their abilities and how to best apply them in my particular circumstances.
Multi-boxing a varied group like this does lead to a large amount of inefficiency, since there are so many abilities that I won't use often or perhaps at all. Then again, that's not entirely surprising, the game is designed around each character in a group being controlled by a separate player.
My leveling path was to get the four characters leveled via quests as a group until level 8, which is the lowest level at which they can zone into Ragefire Chasm. I ran RFC until they were 14, at which point I skipped Wailing Caverns and ran Shadowfang Keep. Wailing Caverns is very long and very confusing and visibility in some areas is poor, which means I may miss a mob or two, which won't aggro until one of the low-level characters gets too close. So I skipped it until recently, when I did a couple of short runs to complete the quest to get 20 Deviate Hides, which provides the characters with 10-slot bags as a reward.
Shadowfang Keep, on the other hand, is perfect for power leveling. It is a short and linear instance with mob groups that are relatively densely packed. So you're not really getting cheated on exp and loot just because it's short. I did all of the quests for Ragefire Chasm, but none for SFK and only one for WC. The run-throughs give good experience to the group, and many of the quests require a series of prerequisite quests that I'd just as soon skip.