It was the practice of the Prophet (peace and blessings be upon him):
"When children were brought to him (peace and blessings be upon him), he would pray for blessing upon them, perform tahnik for them (chewing a date and placing it in the child's mouth), and supplicate for them."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4876