Just make sure you measure carefully and mark any containers you put the powder into carefully, including the recommended dosage, and a warning not to exceed this dosage. Good food sources of magnesium other than green leafy veggies are seeds and nuts if the magnesium was in the soil where it could be absorbed by the plants. The chemical fertilizers commonly used in agriculture often drive magnesium so deep into the soil that the plants can't reach it. And our soils have become depleted. One more common cause of magnesium deficiency is the use of antibiotics.
Antibiotics waste tremendous amounts of magnesium.
And often children with one health challenge also have others, such as allergies and ear infections. If your kids are having lots of ear or respiratory problems, take them completely off dairy products for three weeks and see what happens. It usually takes two weeks for the mucus to clear significantly. You could get an herbal expectorant from a health food store to facilitate this process. Another reason to avoid dairy products is that milk has lots of the antibiotics in it that were fed to the cows.