[Eönwë]

Hobbits are men, not just in sharing the same fate. They are part of the druedain, or pukel-men, like the ones that save the rohirrim i the return of the king (book). So they would go to Mandos, and then to Eru like the redt of men, and like the rest of Men (according to the lost tales) they would sing the new world (or Arda renewed as some here have called it) into being along with the Valar.

Also Eru did know that dwarves would be made. He allowed him to make them though not only because of free will, but because, as he puts it, every act done in or connected to middle earth "redounds in my [and its] glory".

I don't know if Ents do become trees actually, though, because Treebeard (the first and most faithful Ent) doesn't like the Huorns (which were once Ents), which shows that even the old Ents can stay Entish. I think that they just continue to live and be Ents until "the changing of the world" or even Dagor Dagorath.