Goblins (orcs, etc.) reproduce in the same way that humans do. That goes a long way to making them into people--you can have baby goblins and elderly granma goblins, if you wanted.
How do demons reproduce? If you want them to remain inhuman, killable monsters, then have them reproduce as such. They aren't born; they spontaneously generate out of corrupted places or out of piles of filth, spawn from evil thoughts, or are formed out of souls stolen or captured by greater demons. (This is assuming you don't have them being created out of the souls of the damned, as is traditional.)