There is a sudden shift in portrayal in many of these famous fairy tales.
The Disney versions are more fairy-like, colourful and optimistic and child-like, but the originals are often more cruel or even heartbreaking, especially that these stories were meant to have moral themes for the children who were reading them.
Like in the Little Mermaid, the mermaid commits suicide, or in Pinocchio, it was originally intended to end with Pinocchio being hanged for his misbehaviours, or unlike in the Jungle Book, Mowgli kills the tiger and even becomes ostracized by the nearby village and destroys it entirely out of revenge, or Sleeping Beauty was actually raped and impregnated by a monarch while she was sleeping.
Lots of famous fairy tales have this same theme, even the Grimms brothers ones which sounds like unfairy-like.
So how come these original versions where more gruesome even though they were intended to be made for children?
How did these stories shift to a more light-hearted setting as time progressed?
To answer your actual question, earlier fairy tales were not for children. The "fairy tales is only for children" is a recent thing.
There are no "original version" of most fairy tales. In norwegian we have a useful distinction: folkeeventyr og kunsteventyr. More or less folk-tales and constructed tales. Most famous fairy tales are folk tales. Passed down orally for hundreds of generations and changed slowly to fit in the current cultural context. Asbjørnsen and Moe, Brothers Grim, and many others travelled their country, collected and wrote down the tales they heard. Because they were written down they stayed constant, and are still in the same form they were in the early 1800s, and we see them as "original" because of that. Disneys stories are actually keeping with traditions, because they took fairy tales and put them in a form that would fit their current time.
200 years is nothing in the life of a fairy tale. It is impossible to know for sure how old they actually are, but through historical linguistics and tracing common themes, plots and features we find that some stories might have been passed down for over 5000 years. So knowing this, the concept of "original" fairy tales isn’t really a thing. Humans just love stories and magic and telling the stories they know and love to their children.
If you want to read more: https://phys.org/news/2016-01-phylogenetic-analyses-fairy-tales-older.html
Excellent answer by /u/dwarrowdam. I will add to this and tackle the specific examples OP provided.
Folklore - the actual oral tradition of the folk - assume many genres. In addition, folklore has inspired a literary tradition that seems like folklore, but is distinct from oral tradition.
The folktale in Europe (folkeeventyr in Norwegian as /u/darrowdam tells us; Märchen in German) was an elaborate oral narrative told largely for adults. These were oral novels and they could take several nights in the telling. As a genre, they were generally told not to be believed, they were multi-episodic, and they ended happily.
Another genre that is often confused with the folktale is the legend, stories that were generally told to be believed, were single-episodic (generally) and often ended horribly.
The word "fairytale" means different things to different people; as /u/darrowdam tells us, in Norwegian these are kunsteventyr in Norwegian - "art" or "literary" tales. Throughout Europe, these were inspired by the publication of stories of the collections of the Brothers Grimm and others, beginning in the early nineteenth century (earlier for the French Perrault), based largely Märchen - folktales they collected from folk storytellers.
These literary fairytales were inspired by folk oral tradition, but they were the product of single authors and did not adhere to the "rules" or the genres of folklore. Literary fairytales took many forms, but again, because of they were literature, they could end horribly and yet still seem like a folktale.
We see this with "The Little Mermaid" (Danish: Den lille havfrue) by Hans Christian Andersen (1805-1875): mermaids rarely figure in folktales, but they are commonly found in legends - and so many of these narratives end horribly. In keeping with this, Andersen borrowed a motif and embraced it with folktale-like story, producing his (and his alone!) fairytale of "The Little Mermaid."
Pinocchio and the Jungle Book are also stories borrowed from literature - but not from folk oral tradition.
Disney did borrow stories from collections of actual folk tradition and from these authors who were writing their own "fairytales": "Snow White," "Cinderella," and "Sleeping Beauty" - the original "big three" - were each taken from oral folktales, and as such, the original stories ended happily. They were, as indicated, stories told for adults, and they were, as such, occasionally violent and had elements not appropriate for children. The various collectors (Perrault and the Brothers Grimm, for example) tended to "cleanse" and shorten the tales for publication, in part as a process of recording the stories before electronic recording machines, but also because they pitched these book for children. This began the process of perceiving the folktale/Märchen/folkeeventyr as intended for children - even though this was not the case.
I'm not sure about the origin of the idea that "Sleeping Beauty was ... raped ... by a monarch while she was sleeping." I have never seen this motif in an actual folktale. I suspect this, too, is a modern literary adaptation of a folktale. This motif is grossly inappropriate for the folk genre of the folktale.
The history of the literary adaptations of folktales into fairytales and ultimately into Disney films is a bit tangled, but when the various threads are unwoven and set apart, it is clear to see that the process is easily understood.