You can test for zinc deficiency by taking zinc lozenges. If they taste sweet, you are deficient. If or when they start tasting metallic you are good on zinc!
It's also best to eat foods that contain zinc. Seafoods and Shellfish (Oysters!), Grass-fed meat, and eggs are all good sources of zinc.
Zinc is also the "brain food" for fetuses in the early months, and is a great thing to have plenty of in a preconception diet for both mom and dad to be. (And of course through-out pregnancy.)
It's also important for growing kids.
If they are chewing on their nails, it's a sign of "nutrient recycling" It's an urge to get minerals back in their body from their nails. Seek out more nutrient rich foods to help compensate for deficiencies, and help stop the nail biting urge.