Here, I am attempting to track some early occurrences of the motifs in the thumbling narrative. It's impossible to say what the "first" version of any folktale is, but it is ​possible to say "Hey, stories about XYZ things definitely existed at this point." I also include the publication dates of some well-known fairytale collections and some trivia pieces.

​The first thing to know is that there are really two stories to track here: one, the Thumbling Narrative, with a thumb-sized child who gets into trouble or gets swallowed by animals but proves himself; and two, Le Petit Poucet, about a small-ish youngest brother who is abandoned with his siblings but rescues them from a monster. 
  • c. 700 BC: In Hesiod's Theogony, Greek titan Kronos swallows his children but disgorges them alive.
  • 7th-4th century BC: Homeric hymns. The Greek god Hermes is a precocious infant who, like Thumbling,  becomes a trickster and thief on the day he's born. He begins by going out to the fields and driving cattle.
    • Many writers have pointed out similarities to Thumbling. Lithuanian and Albanian tales also have Thumbling steal cattle. Similarly to a Thumbling tale, it features scatological humor. In some translations, when Hermes returns to his mother's cave, he enters through the keyhole - but I wouldn't take this as an indication of his size.
  • 4th century BC: The story of Jonah is recorded. Man is swallowed by fish and emerges safely.
  • c. 711 AD: Sukuna-biko, the tiny Japanese god, makes an appearance in the Kojiki.
  • 720: The Nihon Shoki mentions Sukuna-biko.
  • 1st century AD: Hyginus, Fabulae. The story of Athamas' and Ino's children; a murderess is tricked into killing her own sleeping children when the victims switch clothes with them. This is a central motif to Le Petit Poucet. Another story about Ino's family has the children fleeing their evil stepmother and flying into the wilderness on a golden ram - again, similar to the flight/abandonment in Le Petit Poucet.
  • c. 300 AD: "The Enchanted Brahmin's Son" appears in the Panchatantra.
    • This is a variant of the beastly bridegroom tale, with other examples being Asinarius ("The Donkey Tale," c. 1200), Straparola’s King Pig (1550-1555), the Grimms’ Hans my Hedgehog (1819), and the Norwegian King Lindworm (1889). In some eras, deformities at birth were often believed to be caused by either sexual sin or maternal impression.
    • All of these tales feature parents who long desperately for a child, no matter what it looks like. In The Enchanted Brahmin's Son, a childless brahman prays for a son, only for his wife to give birth to a snake. This is the first known literary example of this motif, which plays a large part in all thumbling tales.
  • 10th century(?):  The Tale of the Bamboo Cutter, Japanese tale. Childless woodcutter discovers the incredibly tiny Princess Kaguya inside a bamboo. The oldest surviving manuscript dates to 1592.
  • Late 10th century: "De quodam piscatore quem ballena absorbuit" (About a Certain Fisherman Whom a Whale Swallowed), a poem by the French monk Letaldus. A fisherman named Within is swallowed by a whale. He kills it and it washes up on a beach. When people hear him shouting inside it, they think the whale is possessed. After he explains, they cut the whale apart and free him. 
    • This is similar to Jonah but is a step closer to Tom Thumb inside the fish, and Thumbling crying out inside the cow's belly; the thumbling sometimes torments or kills the swallower from inside its belly, and must be cut out by outsiders. 
  • 12th century: De Reis van Sinte Brandaen, a Dutch epic, has St. Branden encounter a man one thumb high who is floating on a leaf on the ocean.
  • c. 1210-1214. Otia Imperiale, Gervase of Tilbury. Mentions fairies called portunes, half a thumb in height.
  • 1392–1573: Many short narratives known as the Otogizōshi are written down during the Muromachi period in Japan. These stories include the romance of Issun-Boshi.
  • c. 1517. Egerton 1782 is an Irish vellum manuscript, first compiled by scribes of the Ó Maolconaire family, which features the tale of the Death of King Fergus mac Leti. In this parodic tale, King Iubdhan and Queen Bebo of the tiny leprechaun people come to visit Fergus' court, but in a dramatic scene, Iubhdan falls into a cauldron of porridge. Another leprechaun is popped into a goblet of wine.
    • These scenes are echoed in the English Tom Thumb, who is boiled into a pudding and falls into a bowl of the king's frumenty; the German Daumerling, baked inside a sausage; and Norwegian tales of a thumb-sized child named Tommeliten or Tume, who drowns in buttered porridge.
    • O'Grady, Standish, ed. and trans. Silva Gadelica. ​1892. pg. 276-277.
  • ​1579: William Fulke mentions "a little child like Tom Thumb" in anti-Catholic Heskins Parleament Repealed
  • 1621: "The History of Tom Thumbe" is the first fairy tale printed in English. Original publishing date is unknown and authorship is uncertain; earliest surviving text is a booklet printed in 1621. The foreword makes it clear that Tom Thumb is already a well-known tale. Most of it is unique and focuses on Tom's adventures with King Arthur. See the Tom Thumb timeline.
  • 1634-36: Il Pentamerone is published. 
    • "Nennillo and Nennella" has similarities to Athamas' children, Hansel and Gretel, and Le Petit Poucet. (Also features a child being swallowed by a fish and emerging safely.)
    • "The Myrtle" has a woman wish for a child, even if it's just a sprig of myrtle, and having her wish granted.
  • 1697: Le Petit Poucet appears in Histoires ou contes du temps passé by Charles Perrault. This story is not really a Thumbling tale, but the introduction is reminiscent of one.
  • 1730: Tom Thumb, the play by Henry Fielding.
  • 1744: Tommy Thumb's Pretty Song Book is the first-ever book of English nursery rhymes. 
  • 1776: Hans Holck's chapbook "Svend Tomling" is a Danish Thumbling tale. Although it's literary and embellished, it is generally the same story collected by the Grimms and others: a farmer and his wife have a small son who drives the plow, is sold to a nobleman, escapes from him, falls in with thieves, is swallowed by a cow and speaks from inside it before escaping, and finds his way home.
  • 1784: "I Had a Little Lairdie" (Joseph Ritson, Gammer Gurton's Garland: or, The Nursery Parnassus) - a song with similarities to the thumbling tale.
  • 1812: the Brothers Grimms’ Children's and Household Tales volume 1 includes "Thumbthick" and "Thumbling as Journeyman." Volume 2 has "The Young Giant."
  • 1830: The first American-built steam locomotive is named the Tom Thumb.
  • 1835: Hans Christian Andersen's Tommelise, later translated as Thumbelina, is published.
  • 1838: Birth of Charles Sherwood Stratton, who became world-famous under the stage name General Tom Thumb. Some of his stage shows were based on the stories of Tom Thumb and Hop o' My Thumb. In 1863, when Stratton married his fellow performer Lavinia Warren, a book was produced titled "Tom Thumb's bridal tour; a fairy story," in which the Strattons' honeymoon takes place in Fairyland and they meet various fairytale characters including the fairytale Tom Thumb (referred to as Sir Thomas Hop O'my Thumb).
  • 1841: Norske folkeeventyr by Asbjornsen and Moe includes Doll i' the Grass and Thumbikin.
  • 1910: First version of the Aarne-Thompson classification is published. The Thumbling tale will become known as AT Type 700.
  • 1958: Release of the feature film "Tom Thumb," inspired by the Grimms' Thumbling stories.
  • 1994: Thumbelina is adapted as a full-length animated movie by Don Bluth.
The tombstone of Tom Thumb, 18.5 inches in height, in Tattershall. It reads "T. Thumb, age 101, died 1620."
More resources

Anderson, Graham. Fairytale in the Ancient World. 2000.

Bottigheimer, Ruth. Magic Tales and Fairy Tale Magic. 2014.

Da Silva and Tehrani. Comparative phylogenetic analyses uncover the ancient roots of Indo-European folktales. 2016.
https://www.researchgate.net/publication/271767548_A_phylogenetic_approach_of_mythology_and_its_archaeological_consequences

Ziolkowski, Jan M. Fairytales from Before Fairytales. 2007.