In the Mesh Renderer component you can select if that object will cast and receive shadows. So if there are some 3D objects that you don't want to have a shadow, just disable Shadow Casting and Shadow Receiving in the chosen model’s Mesh Renderer component. (I found that from here.)
Additionally, you may want to increase the ambient lighting and decrease the directional light intensity, which means the light is technically everywhere and will give you the cartoon effect that is desired. I found that from here.