The Enchanted Forest by Hans Malmberg & Gunnel Linde
This was not a childhood book of mine, but one that I came across a few years ago on a book hunting session. It is a story about Tiny, Beth and Peter, and their exciting adventures in the land of the Enchanted Forest. They become small by crawling through the cat flap (on the cat's advice) and go searching for their mother's rings.
They encounter many forest animals and birds along the way, and there is a happy end of course - the rings are found and the children grow to their normal size again. The pictures are fascinating - photo montage done years before Photoshop - and they are sometime cute, sometimes just weird, but amusing.