Do you know Gyo Fujikawa? She wrote and illustrated more than fifty books for children between 1963 and 1990, and although I'm not sure she qualifies as classic in the Generally Accepted Canon of Kid Lit, she's way up there on my personal list. I had one of her books (Come Follow Me) when I was little, and I loved it so much--treasured it, actually--that while I was working at a bookstore in college I snatched up every one of her books that came through our bargain section (because that's what I could afford as a poor college student working in a bookstore).
The writing itself...isn't that great, honestly. Some of the stories are quite sweet, and they do include things like manners and values and whatnot, but a lot of the books are a kind of random assortment of rhymes, tales, and disjointed one-liners, and the content doesn't really have much of an narrative arc.
BUT! The illustrations are so wonderful they more than make up for the less-than-brilliant writing.
Aside from featuring adorable kids of all types, the drawings are chock full of the kinds of details that will keep your kids' attentions for hours, whether you ask them questions about what they're seeing ("How many kittens are in this picture?" "What kinds of fruit are being served at this breakfast?") or let them just flip through the pages by themselves. These are my favorite books to bring on plane trips because I only have to bring one and we never run out of things to look at. The illos are full of kids doing fun kid things (dressing up, playing with toys, being silly with their friends), and the backgrounds often feature tiny critters in the foliage or fairies peeking in windows--stuff most kids get a real kick out of finding.
I have such strong memories of the magic of these drawings, and my son is already a big fan too. Our favorites are Oh, What a Busy Day! and A to Z Picture Book. (I could spend 15 minutes on the "B is for babies" page alone.)