I've never had any first hand experience with a seedeater, but please hear me out. I've developed a theory of the origins of seedeaters. I only have the content from your blog to work with, but I don't think seedeaters are born or hatched. I think they are created. This would explain why there hasn't been a child-sized seedeater seen.
I'm going to start by talking about the post entitled "From Jean." I am also going to assume that the disappearance of Craig was indeed the work of a seedeater. Due to knowledge of previous blog posts, I am aware that seedeaters' diets consist at least partially of... well... children. I found the disappearance of Craig to be quite odd. What was the seedeater's motive? What reason did it have to take Craig? After three days of searching, the only signs of Craig to be found was his glasses and jacket. There was no half-eaten corpse laying around somewhere.
My theory is the seedeaters take people and somehow change them into new seedeaters. After discussing my theory with a friend, we came up with some sort of fungal or bacterial infection which could cause mutation. The infection could cause the new seedeater's flesh to rot, which would explain both the smell commonly described as garbage or rotting meat. This would also explain their rag-faced appearance. If their flesh was rotting, they'd need some way to prevent themselves from completely falling apart; which could be accomplished by stitching rags (or pieces of their own flesh) together. Abductions happen in the world every day. Not all of them are explained. Some unexplained disappearances could be the work of seedeaters.
Now you may be thinking, "If seedeaters can change anybody into a seedeater, why aren't there more of them?" I think that the process of changing someone into a seedeater could be very dangerous to the seedeater which does the changing. A seedeater would probably not take someone until the seedeater was already towards the end of its lifespan in an act of self-preservation. If the process of changing a human into a seedeater resulted in a very high chance of seedeater death, then this would result in a very small increase of total seedeaters in the world over a long period of time since on rare occasion a seedeater could survive the process of changing another and go on to change a second human.
Now like I said, I've never seen a seedeater, and I hope I don't. I only have the information you've provided me with, but I thought you might be interested in my theory. Just something to think about."
This is an interesting theory. In this, there may be more than one and they may operate similar to the classic "werewolf" image. Very interesting indeed.