Children's Books

It should be noted that many of these are really more for young children (3-5). I also think a collection of simple science and nature books can be nice once kids are starting to learn to read and learn animals. Getting them invested in kid's science shows and Sesame Street style shows helps a lot also.

Where the Wild Things Are
The Very Hungry Caterpillar
Most Dr.Seuss
The Giving Tree (a personal favorite)
If You Give a Mouse a Cookie
The Velveteen Rabbit
Alexander and the Terrible, Horrible, No Good, Very Bad Day
Clifford books
The Magic School Bus books
Mr. Popper's Penguins
Cloudy with a Chance of Meatballs