SB 7.5.21

āropyāṅkam avaghrāya
 mūrdhany aśru-kalāmbubhiḥ
āsiñcan vikasad-vaktram
 idam āha yudhiṣṭhira
Word for word: 
āropya — placing; aṅkam — on the lap; avaghrāya mūrdhani — smelling his head; aśru — of tears; kalā-ambubhiḥ — with water from drops; āsiñcan — moistening; vikasat-vaktram — his smiling face; idam — this; āha — said; yudhiṣṭhira — O Mahārāja Yudhiṣṭhira.
Translation: 
Nārada Muni continued: My dear King Yudhiṣṭhira, Hiraṇyakaśipu seated Prahlāda Mahārāja on his lap and began smelling his head. With affectionate tears gliding down from his eyes and moistening the child’s smiling face, he spoke to his son as follows.
Purport: 

If a child or disciple falls at the feet of the father or spiritual master, the superior responds by smelling the head of the subordinate.