What makes a color feel 'children's book'?

I’ve been staring at a palette for a new manuscript—bright but not neon, warm but not sickly. The client wants 'whimsical,' but I keep thinking: is it the saturation? The contrast? Or something deeper—like how certain blues feel like afternoons in a sunlit kitchen, or how yellow only works if it’s slightly dusty? I can’t quite name it, but I know when I’ve hit it. What’s your gut sense? What colors make you think of stories that aren’t meant to be taken seriously?