object stretching when i import from blender to unity

The child objects are taking on the parents scale transforms, at least that's what I think is happening. If I had to guess without seeing your .blend file, is that you need to Ctrl-A apply scale transform in Blender to all of your parts (perhaps just apply all 3 transforms). Curious as to why your mesh isn't just one solid object. I understand you may have reasons underlying in your game mechanics but most meshes of single objects don't have to have relationships with the actual mesh containing objects (or the game object containing the mesh renderers), but an empty game object.