In which of those legends and fairy tales were these chaos monsters presented as sentient mortal creatures that mate and bear noncombatant children to be slaughtered by the heros, as they are in KotB? Where in Tolkien's published works (which you reference) were there caves filled with Orc children huddling next to their mothers? As one of those people who clearly doesn't possess the encyclopedic UNDERSTANDING that you have of the folklore informing this adventure, I look forward to the relevant examples you can provide.
I posted a little above on this.
You ask several questions there, I don't know if I have easy answers for all of them, but I'll attempt to answer them as I can.
As I mentioned above, I don't recall any fairytales where they presented Goblins (or Ogres or Trolls) with Children off the top of my head. There may be a story or two of the Goblin King (I seem to recall one or two may have) having a son or child, but i'd have to go through quite a big of books to find which story it was (The most plentiful source of stories I'd say were the Fairy Books by Andrew Lang, these were normally color coded as things such as the Red Fairy Book, the Blue Fairy Book, the Green Fairy book [and my personal favorite], and many more). Grimms Fairy Tales are normally a little bit more simplistic, but Goblins are generally more from English folklore rather than Germanic or other European stories and legends.
Interestingly enough, it was more the Elves and Fairies themselves which may have had children. Elves were NOT like D&D elves generally, they ranged in size and shape and demeanor. Sometimes they were good and sometimes they were very evil. They were far more likely to be married or have children than other creatures of lore. Goblins could be seen as evil elves (for example, Hobgoblins are more of a non-english goblin in origin, but it's basically a term for a Dark Elf...but NOT how we understand Dark Elves to be in D&D).
Normally, as Goblins were the monsters, anything outside of that wasn't really touched upon. They generally were there to torment or hurt the protagonist of the story. Though generally sentient to talk, torment, and threaten, they didn't have children in Fairytales and Lore as a general rule. It's like asking if a Vampire has Children after it becomes a Vampire. Monsters having children is more of a more recent evolution of ideas.
Which brings us to Tolkien. I suppose the first instance of Goblins in caves would be in The Hobbit. There wasn't really much about any children or anything at all about families in that book with Goblins. Goblins were more of creatures that threatened the protagonists of the story and then rode out to battle against the armies of Men, Elves, and Dwarves.
Later, in LotR and other works we learn about Orcs and Goblins, but most of the earlier publications do not touch upon anything with families or even really their origins. It's not until his son Christopher started doing editing and putting out more papers and such that we find out more about the ideas behind the ecology of the Orc and the Goblin.
Of these, only the Silmarillion probably were early enough to influence anything Arneson and Gygax put out, and even that was probably not too influential. It is far more likely that it was basically just the LotR and the Hobbit which influenced D&D.
In that light, the only thing we could have basis for Goblin babies would be a reference or two in other fairystories about anything dealing with Monsters and their babies and children. That was...as I mentioned before...rare. In some instances, when we do see monster babies they are predecessors of an even greater evil of their parent.
One instance of this that you MAY have heard of (and I am sorry if you have not, I'm really trying to go to what I would think is very general information, but from what I've seen in this thread, what I have supposed was common, and WAS common when I was young, MAY NOT BE COMMON KNOWLEDGE anymore...so I apologize if I am jumping on a guess right now that you may have heard about this one...if not, once again I apologize) is found in the story of Beowulf.
In this story you have a Monster called Grendal that eats men. Beowulf is the hero that defeats this monster...BUT...this monster is but the child of it's mother...which than also Beowulf has to defeat.
The story doesn't paint Grendal as a sympathetic creature, but many of the stories and tales of old didn't paint monsters as such. Many times they were there to teach a moral, encourage others to be good, or to show the epic greatness of a people.
So, hopefully that can answer your questions. To give a much fuller answer would literally take a LOT more, and I've already written quite a bit here. I apologize that to type a more full answer would take such a large amount of space, but hopefully this short answer at least gives a small bit of information that can somewhat answer your question.
