ātma-vat suhṛd ucyate
One who is neutral may be avoided like an enemy, but a friend should be considered like one’s own self.
Even if Nanda Mahārāja did not see friends, enemies and neutral parties as entirely equal, Lord Kṛṣṇa, being Nanda Mahārāja’s son, was certainly a most trustworthy friend and should therefore not be left out of intimate discussions. In other words, Nanda Mahārāja might have thought that as a householder he could not act on the highest saintly platform, and thus Lord Kṛṣṇa furnished additional reasons why His father should trust Him and reveal the entire purpose of the sacrifice.
According to Śrīla Jīva Gosvāmī, Nanda Mahārāja stood silent, doubting his position of parental aloofness, since Garga Muni had predicted that his son would be “equal to Nārāyaṇa in His qualities,” and the young boy had already conquered and killed many powerful demons.