What childrens book in the featured a tiger Rousseau style paintings?

You're likely thinking of "The Tiger Who Came to Tea" by Judith Kerr. While the illustrations are not technically in the style of Henri Rousseau, they do have a similar feel, with their flat, bold colours and simplified forms. The book features a tiger with large, expressive eyes and a somewhat whimsical, almost surreal, appearance, which echoes the style of Rousseau's jungle scenes.

It's worth noting that the illustrations in "The Tiger Who Came to Tea" are done in a style influenced by several artistic movements, including Art Deco and children's book illustration of the early 20th century. However, the playful and slightly fantastical nature of the tiger's design certainly resonates with Rousseau's unique style.