The Great Big Book of Families by Mary Hoffman and Illustrated by Ros Asquith is one of my all time favorite books for kids. This book demonstrates that families comes in all shapes, sizes, and configurations. Families live in all kinds of situations. Families come from all kinds of cultures and backgrounds. Families experience all kinds of feelings, and do all sorts of things. If you are looking for a book that shows the diversity of people and families while still holding a thread of commonality and unity look no further. I recommend this book for toddlers on up. My almost 4 year old loves this book and it has sparked many interesting and fruitful curiosities and conversations. She can look at the detailed pictures for long stretches of time.
Standing On My Own Two Feet: A child's affirmation of love in the midst of divorce by Tamara Schmitz is a lovely book for a young child that has divorced parents (I'd recommend to children under the age of 6). The story very matter of factly shows how the child, Addison, has two homes, and sometimes misses the parent he isn't with. It reinforces that the reason the parents don't live together anymore isn't Addison's fault. And empathically states that Addison is loved, no matter what. It is a very upbeat and positive take on a young child's experience with divorce.