udāsīno 'ri-vad varjya
ātma-vat suhṛd ucyate
udasinah -- one who is indifferent; ari-vat -- just like an enemy; varjyah -- is to be avoided; atma-vat -- like one's own self; suhrit -- a friend; ucyate -- is said to be.
One who is neutral may be avoided like an enemy, but a friend should be considered like one's own self.
Even if Nanda Maharaja did not see friends, enemies and neutral parties as entirely equal, Lord Krishna, being Nanda Maharaja's son, was certainly a most trustworthy friend and should therefore not be left out of intimate discussions. In other words, Nanda Maharaja might have thought that as a householder he could not act on the highest saintly platform, and thus Lord Krishna furnished additional reasons why His father should trust Him and reveal the entire purpose of the sacrifice.
According to Srila Jiva Gosvami, Nanda Maharaja stood silent, doubting his position of parental aloofness, since Garga Muni had predicted that his son would be "equal to Narayana in His qualities," and the young boy had already conquered and killed many powerful demons.
