I feel like I've seen a lot of examples of naive characters like you described, but I'm drawing a blank on examples. I would consider the mouse in THE LITTLE MOUSE, THE RED RIPE STRAWBERRY, AND THE BIG HUNGRY BEAR to be naive, although it's the narration that shows this and not the illustrations. (The narrator is talking to the mouse and you as the reader know the mouse is being tricked.) NOTHING EVER HAPPENS ON MY BLOCK by Ellen Raskin is a great example of the main character being oblivious to what's going on around him in the illustrations, but that's an older picture book. (Short text though, I think it would still get published in today's market.) I'll keep brainstorming for more modern titles. . .