You can apply a 'Normals' operator on your meshes to give a smooth appearance while maintaining the low poly count. To do so, right click on each mesh node in your scenegraph on the left and click "Convert To --> PolyMesh". Your meshes are currently in "BinMesh" form (indicated by the blue icon next to it), which is a simple geometry node but can not be manipulated. By converting to a "PolyMesh" node (will be a grey icon), you can now apply various operators to modify the mesh. Once you have done this, select a mesh, and in the right hand panel under the 'PolyMesh' plug, click the 'Add Operator' dropdown selection, and select 'Normals'. Under the options for the Normals operator, set 'Normal Type' to 'Auto Smooth', which is usually a decent choice. This will modify the lighting on the polygons to simulate smooth surfaces, even if the actual polygons have sharp edges between them. Then re-render to see the result.
Hope that helps!