Update/copy my posts to all other sub-sites

I know it is a little bit weird, but I want to copy posts from a demo site to other selected subsites, and i may do this very often. Those copied posts can be on a separate page or in a separate category but looks like they are written by each sub-stie owner. So how to make this happen? Any suggestions? Of course, if this can be done automatically, but some manual work is also acceptable.