Your kids are going to try chewing their nails. And even though you parents do it too, you'll feel obliged to discourage your children from the habit.
Ethan Millard and Brian Foxx have some ammunition for you:
You can spread disease. You're putting unwashed fingers in your mouth!
You can also damage the tissue around your nail and that can lead to a gross infection.
You can also spread things like warts across your fingers.
It's also socially damaging. People get grossed out by it.