When doctors vaccinate a patient, their intention is to expose him or her to a weakened form of a disease-causing pathogen and thus to make the patient better able to resist the pathogen and less likely to develop a severe form of that disease later.
The statement 'Some parents read their children fairy tales containing allegorical treatments of treachery and cruelty, with the intention of making them less emotionally vulnerable to these phenomena when they encounter them later in life' best illustrates the principle that the passage illustrates.