I love the fantastic books made by the Danish art historian and artist Egon Mathiesen back in the 40's and 50's. They are simple and poetic in both illustrations and texts. "Mis med de blå øjne" ("The Blue-eyed Pussy") was the first book that really captured Frida Viola - because of its strong simplicity, I guess. Egon Mathiesen's project was precisely to make art for children.
We found "Fredrik med bilen" ("Fredrik with the car") at the flea market - and it is already a favorite. It was first published in 1944 and it has been in the bookshops continuously ever since. The story is about Fredrik who travels around all of the world in his dad's yellow car - meeting all kinds of nationalities and finally realizing that we are all the same despite the colour of our skin.
There is a nice article on Egon Mathiesen with more illustrations at Danish Literary Magazine.