We’ve all heard that milk does the body good, especially for growing children who need the calcium to build strong bones. But a new study shows that Popeye may have had the right idea with his spinach habit.

Researchers at Baylor College of Medicine in Houston found that foods high in magnesium, such as spinach, may be just as important for bone density in children as calcium. Previously, magnesium was found to be a key nutrient for adult bone health, but the results from this study showed that magnesium is just as important as calcium in a child’s diet as well.

So what should parents be sure to pack in their child’s lunchbox to make sure they are getting enough magnesium? In general, the National Institutes of Health says that green vegetables are a good source. The center of the chlorophyll molecule, which gives veggies their green color, contains magnesium. Nuts, seeds and whole grains are also a good idea. However, processed or refined white grains are low in magnesium, so be sure to distinguish between the two when trying to pick out trail mix or granola bars. Kid-friendly foods that contain magnesium also include peanut butter, chocolate pudding, chocolate milk and yogurt.

But, greater bone density is not the only benefit of this nutrient. Magnesium also helps maintain normal nerve and muscle functions and keeps the immune system healthy. A magnesium deficiency could cause fatigue and weakness. Researchers say it’s important to make sure your children are getting all of their required nutrients by keeping meals balanced. For example, try substituting their sugar-packed cereals with whole grain varieties. That is just one of the many ways to make sure they’re getting both calcium and magnesium.