The theme of a book is an ‘abstract idea’ such as friendship, loyalty or a quest for identity.
Themes are different to morals. The theme should be subtle and strengthen your story by adding depth and meaning. Visit your local library and look at some of the classic picture books. Notice that although the text may be brief and tightly written there lay truths that are timeless. The story should leave readers with a residual feeling that stays with them.
You should be able to sum up your theme in one or two words. The theme of the first series of Season books is friendship. I have written two more Season picture book series since then. This is a book with a compilation of my Animal Stories for all Seasons. They will also be available to buy individually.
Your characters should always carry the theme. It is important not to blurt out your theme but to let it emerge from the story. If you must come out and say it, do it in dialogue, not narration and whatever you do avoid preaching. Children’s stories should be explorations of life not Sunday school lessons.
In picture book you should keep your theme positive. If writing about difficult situations and feelings, offer constructive ways for your readers to cope with them.