Dark leafy vegetables are very good for your children because vitamins A, C, K, folate, iron, and calcium are present in these greens. My mentors have told me that these are high in fat-soluble vitamins, so in order for these to work, you have to cook it in fat like butter or olive oil.
Saute a piece of onion and crushed garlic cloves in olive oil or butter until translucent.
Toss in your washed spinach leaves. If you're cooking a hefty bunch, try to saute them in batches so that they will all cook evenly. Then season with salt, pepper, and nutmeg.
All these ingredients are readily available in any supermarket.