The book goes into quite a lot of detail about the source of Treebeard's knowledge of the world around him. Ents remember all the (good) speaking races by means of a rhyming list - and hobbits aren't on it:
Learn now the lore of Living Creatures!
First name the four, the free peoples:
Eldest of all, the elf-children;
Dwarf the delver, dark are his houses;
Ent the earthborn, old as mountains;
Man the mortal, master of horses:
In fact, it's perfectly natural for them not to be on it, because they have been keeping themselves to themselves for as long as anyone knows. The events of the Lord of the Rings are the first time they have involved themselves at all in the world around them[1].
In fact, once Treebeard and the Ents get to know Merry and Pippin, they agree to add a line to the list to record their existence:
Ents the earthborn, old as mountains,
the wide-walkers, water drinking;
and hungry as hunters, the Hobbit children,
the laughing-folk, the little people
[1] with the exception of their (claimed) participation in the last battle against the Witch-King of Angmar: "they sent some bowmen to the aid of the king, or so they maintained, though no tales of Men record it."
Note on origins, as requested in comment
We don't know the details of the origins of either Hobbits or Orcs. The prologue to LotR just says about Hobbits:
It is plain indeed that in spite of later estrangement Hobbits are relatives of ours: far nearer to us than Elves, or even than Dwarves. [...] But what exactly our relationship is can no longer be discovered. The beginning of Hobbits lies far back in the Elder Days that are now lost and forgotten. [...] Yet it is clear that Hobbits had, in fact, lived quietly in Middle-earth for many long years before other folk even became aware of them.
We do know more about Orcs, from the Silmarillion. They were created by Melkor, Sauron's original boss, back in the very beginnings of the First Age - either (it's unclear) in mockery of Elves, or by corrupting existing Elves.
