Children's book about a child who becomes the size of an ant and bird to learn about their world

[Heather]

Probably published in England. Read around 1946 as a school book. Gorgeous black and while illustrations. The child became the size of all the various animals she/he learned about.

[Ayshe]

Probably the book that is searched for here: https://www.goodreads.com/topic/show/...
https://www.goodreads.com/topic/show/...

[Ayshe]

This appears to be Australian, but maybe Wondercap Adventures with the Sandman by Muriel Olyott?

A child wonders what it would be like to be a certain animal, and when they wear the 'Wondercap', they become that animal. Successively, Judy becomes a chick, an ant and a mermaid, while John becomes a lion and a sheep dog. Each adventure was designed to be informative and to teach the children respect for all forms of life, even imaginary mermaids.
From Bottersnikes and Other Lost Things: A Celebration of Australian Illustrated Children's Books

[Pamela]

This also sounds like The Sword in the Stone, which was part of T. H. White's The Once and Future King. The Sword in the Stone

The Once and Future King

Although in T. H. White's version, the boy actually became a bird and an ant.

[Rainbowheart]

Still looking, Heather?