1. Remove the spinach from the box and let it defrost in the container of a food processor or a blender for about 20 minutes. (The spinach needn't be totally defrosted, but if you cut the block into 4 pieces, it'll be easier to purée.)
2. Add chicken broth, chives, yogurt, salt and pepper, and nutmeg; puree until smooth and ice chunks from the spinach have been broken up. Pour into a serving bowl, and add salt, pepper, and nutmeg to taste.
3. Serve the soup ice-cold in individual bowls, topped with a dollop of yogurt and a sprinkling of chives if you like.
How kids can help: Add yogurt and chives to each soup bowl.