The phrase "Sharper than a serpent's tooth it is to have a thankless child" expresses the deep pain and bitterness caused by a child's ingratitude. It means that having a child who shows no appreciation or gratitude can hurt a parent more than a snake’s sharp fangs hurt. This line is from Shakespeare’s play King Lear (Act 1, Scene 4), spoken by King Lear to convey the emotional torment of being unappreciated by his own offspring.
