Harold and the Purple Crayon by Crockett Johnson.

First published in 1955, this little book is still fun today.

Harold is a 4 year old who discovers that he can alter his world with a purple crayon. He uses his crayon to create things in his mind, leading him on adventures with a dragon, a sailboat on the ocean, a hot air balloon... and more!

The focus on imagination is important, especially in today's world where children seem to use their imaginations less and less.