We started with Minimus and Minimus Secundus, both of which DS really enjoyed around that age. You do have to get the teacher's guide, though... that's where most of the activities are. Unfortunately it's priced for a teacher who plans to use it frequently, so it's not cheap!
After Minimus we went on to Lingua Latina, and have really liked that. It is challenging... very very challenging... but the hard work pays off, and you really do start out reading real Latin right away, which is very cool. For that one you need the text, the exercise book and the College Companion (which combines several of the other grammar reference materials in one volume). We also got the teacher's edition of the exercise book so I could check his work more easily (not being fluent myself...) and an extra reader just for fun.
