Style of Beatrix Potter is, according to my opinion very adorable mostly because she used child's mind when she was creating.  She using anthropomorphism where animals having human characteristic and behave like humans. For children is a great way how to identify with a character. Eg.: Peter's blue coat perfectly fits which show Potter's knowledge about animals anatomy, because it is convincing. Her observation experience from her childhood allows her to draw the character in many positions, whether it's standing, laying, is dressed ... Because the book is quite small she is not using a whole page illustration but she keep the character on its own - without background- sometimes she fade away the edges and sometimes she is using a square shape that will fill with illustration. but based on the amount of details you can see what is meant to be centrepoint of reader's attention.

Beatrix Potter started all of her illustrations as sketches into her sketchbook, then she used a pen to create the line work and later on over that  she added watercolors. She working with watercolors very carefully to create the thin and soft layer. She wanted to have all the attention on the main character, that's why she put all the action, costumes into it and the background is not having a lot of details, bare line work, fewer details.