K.O.
This sounds like a typical transition out of napping, although a little early. The transition will be hard no matter what age they start. My best advice is to at least have some sort of quiet time, whether you make them just lay in their bed and "look" at books, or put on a little movie, or some quiet music for them. The main thing they need is a break from too much activity and stimulation. This should make those crabby evenings easier to deal with. I have two kids, 6 and 9 and it worked wonderful for both of them during their transition out of napping. If you make it a "choice" for them, napping or quiet time, chances are they'll pick quiet time, and sometimes will just fall asleep anyway! Good Luck!