I read this about 1950, and I'd guess it was published in the 1940s, probably postwar. A child visits a zoo with all alien critters; I believe it's set on earth, but no guarantees. There's no particular story line, the attraction is a bunch of color drawings of aliens in the zoo. There's no kicker at the end that I recall, just the bunch of drawings. This was aimed at a VERY young audience, certainly under 10 years old. A pleasant little family story about a day at the zoo. English language. Size probably 9 or 10" tall pages. Thin. Gaudy cover.
This is a long shot because it's not a children's novel, it's a 400-page hardcover science fiction anthology with 5.5" × 8.5" pages. However, there is a section in the front (on pp. 31–52) which is almost exactly like what you described. I wonder if it could have been published separately as a children's book? Do these pictures ring any bells?
The book is Travelers of Space, edited by Martin Greenberg and first published by Gnome Press in 1951. The feature matching your description is a set of 16 full page color illustrations of aliens by Edd Cartier (ISFDB, Wikipedia, Pulp Artists, Tellers of Weird Tales) titled "Life on Other Worlds" with an accompanying text story "The Interstellar Zoo" by David Kyle.
From the ISFDB:
According to the contents page "The Interstellar Zoo" is a "special descriptive story" written to accompany Edd Cartier's illustrations (see next note). The text of the story is found on pages 31-32 and 49-52. The title "Life on Other Worlds" refers to 16 pages of full color illustrations by Edd Cartier found on pages 33-48. That title, which is the only title found in the book for this section, was printed solely on the contents page.
Cartier's pictures are available at Monster Brains.
Kyle's story tells of a family's visit to the interstellar zoo, where they view the alien beings through one-way glass and read about them in a booklet. Most of the exhibits are intelligent and are there as volunteers.
"'Picture Number One,'" Mrs. Murray read, "'shows a Venusian.'" (Page 33) "'In the cloudy atmosphere of Venus, this 50-foot being constantly floats above the semi-liquid surface of the planet. All its sense organs, including its brain, hang beneath its balloon body. The long, sensitive tentacles are remarkably dextrous—'"
"Excuse me, Mother," said Don, "but it's not necessary to read it aloud. I know all these simple facts."
"Let's not forget Harriet," his mother replied patiently. "She's still a little girl and can't read very well."
"That's all right. I'll explain things to her."
Don took his sister by one hand and rubbed her frizzly blonde hair with the other.
"They have to live inside that big case, Sis, because the air is thick and specially mixed just like on Venus. And they can't stand direct sunlight so the walls are tinted—which is why they're so difficult to see. . . ."
Harriet dragged him on to the next section. The huge white figure inside was clearly seen. (Page 34)
"Big bean!" Marion exclaimed.
"—Being," Don corrected. "And it's big, all right. Probably thirty feet tall. The air is blue because it's more like ammonia than anything else. He's galactic—not from our solar system. Look at its trunk—it uses it like a hand."
"Like an elephant," Mrs. Murray added helpfully.
"That's not a good comparison," Don said, rather stiffly. "Much more like a hand. You'll note that every intelligent being has some sort of hand, or at least a hand substitute. It's a mark of intelligence—any such being almost invariably has to have a mechanical means for manipulating its environment."
There is a small twist at the end when little Harriet says:
"I know what I want to be when I grow up, Momma," she said.
"In a zoo. In a zoo on another world!"
Don laughed, but Mrs. Murray didn't—the idea intrigued her woman's vanity.