τέκνον teknon and παῖς pais are used fairly similarly throughout Classical and Koine Greek: both mean ‘child’ in the plain sense. Often pais is reserved for people who are absolutely young, such as babies and toddlers. Teknon, on the other hand, can be applied to anyone who is relatively younger: an 80 year old can address his 55 year old son with φίλε τέκνον phile teknon (‘my dear son’).