Was the mother in your possession for a long time or did you just get the mother shortly before you tested her? It can take a month or two, even three after the cat in infected before the test accurately comes up positive. So go off that. You might want to have her retested. To answer your original question though, it is safe to just test the mother, the kittens should 99.9% of the time have the same results as the parents.
If she does ever turn up positive though, then I would go ahead and test the babies just to be on the safe side and be sure. If they have only been around her, there is no other way they could likely get it.