Children's novel about beasts

[SCF]

I read this book in the late 70's but I had the impression that it was published much earlier, perhaps even in the 40's or 30's, but I could be completely wrong about that. I remember the plot, and it had pen and ink illustrations. The plot is about a girl, about 10 years old, who is somehow transported to somewhere else. This other place has giant plants and bugs, as if she has really shrunk, but it also has plenty of things normal size too. She meets up with various bugs, one of which is a catipillar and they all journey on some sort of quest. The land is ruled by the "Beasts" who in the illustrations looked a lot like warthogs and are about the size of horses comapred to the girl. At some point they decide that they need to infilterate the beast's compound to gain more information to aid their efforts to defeat the beasts. They rescue a trapped canary (small in relation to the girl, like a normal bird) from the beasts and the girl takes its place, telling the chief Beast that she is the canary who has changed herself by magic in order to sing more pleasingly.
Do you know the Name and Auther of this book?

[HomieBear]

No but I totally want to read it when someone figures out the title.