I wrote earlier about looking online for information on Japanese detective stories. For children's books, the supply is in some sense worse; but there are a couple of very good sites.
I haven't found any English language blog devoted to reviewing Japanese children's books, at least none that had more than one or two posts. There is a group blog of some translators of Japanese children's books, SCBWI Japan Translation Group. It has some posts that could be of interest for people who want to find more about Japanese children's books, particularly in this category.
The International Institute for Children's Literature, Osaka, apparently no longer exists as such, having been disbanded in 2009 and incorporated into another library. Its website still exists; and there is a really useful list of a hundred representative works from 1868 to 1979, with descriptions and sample illustrations for each book. The list is divided into two parts, 1868-1945 and 1946-1979. There is an English version; but you have to go down from the top level into each half before the button for it appears: 1868-1945; 1946-1979. The enlarge feature for the illustrations doesn't seem to work on all the English pages; you might want to try clicking over to the Japanese in that case.
JBBY, the Japanese Board on Books for Young People, has a little information, and unlike the IICLO it includes more recent books.
J'Lit, the site of the Japanese Literature Publishing and Promotion Center, has pages describing authors and representative works. It has some older works, but the emphasis is on current and recent writers. The author descriptions often mention books that have been translated. They have a category for children's books.
There's one author specific site I know of, The World of Kenji Miyazawa, devoted to Japan's most famous children's writer.
The page by Satoru Saito at Columbia with bibliography on popular literature, that I mentioned on the page on detective stories, also has a section for secondary literature on children's books.
Aozora Bunko has a lot of children's literature that is in the public domain and can be downloaded. I'll just name a few here, putting the Aozora Bunko link in brackets. Two writers in particular are known as children's writers. KENJI Miyazawa (宮沢 賢治), who also wrote poetry, is probably Japan's most famous children's writer. I find some of his stories are payment enough for the trouble of learning Japanese, for their vividness, liveliness and complex simplicity. NIIMI Nankichi (新美 南吉) is also well known. Most Japanese adults seem to have read 'Gon the Fox' in school. (This probably started a teacher experimenting to see if they could get the whole class crying inconsolably in one moment.) Most of the stories by him that I have read seem very simple; sometimes one feels a little uncertain whether the simple moral that the story offers is really all that the story is saying. There are also writers more famous for literature for adults, such as ARISHIMA Takeo (有島 武郎; I translated a couple of his stories here and here), and DAZAI Osamu (太宰 治).
The International Library of Children's Literature in Tokyo is (as you'd guess) a library with a specialist children's literature collection. As major libraries often do, they have exhibitions of books, which might be worth a visit, if you're interested in children's books and you have a rainy day in Tokyo. When I was there last autumn, they had a general history exhibition and one on a particular writer (which I skipped). The general exhibition had a useful brochure with bibliography and links to websites.