The "Dart" in the PHB is the equivalent of the small spears/arrows thrown by an atlatl. I would suggest treating the atlatl as a thrown weapon (5 range incremements max, str applies to damage), but double its range increment (to 40') and increase the critical to x3 to represent the greater amount of kinetic energy imparted to the dart by the throwing tool.
The atlatl/dart combo should probably be a martial weapon instead of a simple one, or even exotic if the PCs are mainly from cultures that don't use such things.