SFK is great for boosting. Take them there at 10 and just leave them at the beginning next to the portal. You can run the entire instance and they will gain XP the entire time without having to move.

The first time through, I go all the way to the end (to get the achievement). After that, I only run to the wolf (Fenrus?) since there's only about 10-15 mobs left in the instance and 1 door that you have to wait for. Time = XP, so waiting for that last door and a handful of mobs is not worth it in my opinion.

When they ding 20, move on to SM.