I don’t have an LZW36 to confirm, but I suspect the default level would be for the light only. I wouldn’t think you would need that functionality for the fan. Granted as I write this I can think of a few cases that might be handy…
This should be fairly easy to test. Create the child device and set a different level. Test if it affects the fan and/or light levels. Remember you can turn off the child device to restore the normal functionality, so other than time it’s a harmless test.