The Dialogues of St. Gregory the Great, but nothing seemed to fit this image I found, painted by Goya in the late 18th century.
This quote, on the contrary, really seemed to work well. It was written as a remonstrance to Theodolinda, the Queen of the Lombards, after news had reached Gregory that the queen had broken off communion with the Church because some rogue bishops had told her that the Roman Church had renounced the Council of Chalcedon.
In the modern context, this quote made me reflect on the plight of parents who endeavor to raise virtuous, high-character Christian children, only to see them snatched away by the false promises of the world. Indeed, one can clearly hear in Gregory's rebuke the stern but loving tone of a father attempting to dissuade his spiritual daughter from the error of her ways. His careworn and patient appearance in Goya's painting meshes nicely with this sentiment.
The full text of Gregory's letter may be found here: