What a lullaby needs that other songs do not
A lullaby is the one genre where excitement is a defect. The features that make a pop song work, a big chorus, a drop, a key change, are exactly what wakes a drowsy toddler. So the prompt should push the opposite way: slow tempo, narrow dynamic range, no drums or only a brushed pulse, and instruments with soft attacks such as music box, harp, nylon guitar, celesta and hummed voice.
Traditional lullabies across cultures share a rocking feel, usually 6/8 or 3/4, at roughly the speed of a slow walk or a gentle sway, between 60 and 80 beats per minute. That is not a coincidence: it matches the motion parents make when they hold a baby. Asking for a rocking 6/8 feel is the single most effective instruction you can add.