If you want to do the splitting of the cube while the game is running, you won’t be able to use anything out of the Blender Python API. Python only has power inside of blender to setup your scene before the game runs. Once the game is build and run, only Haxe ( and JavaScript on some platforms ) can run.
Splitting the cube would require modifying the mesh at runtime using Haxe, like the PhysicsBreak example that @Simonrazer mentioned, but I haven’t done it myself and I don’t know exactly how to do it. Another example that might be useful is the mesh generation example.