I have my doubts about the ‘commentary’ Peter cited, for the simple reason (a thing yet mentioned by Perry Webb) Assyrian dialect (a form of Akkadian language) did possess these specific words. For ‘right’ they had IMNU (also IMITTU); for ‘left’ thay had ŠUMELU (compare also šamal [Arabic], semala [Syrian], and ŠMAL [Ugaritic]).
In every case, getting to the heart of the matter, the expression’s understanding at issue doesn’t refer to children or babies, possessing no life experience, but, more probable, to persons in general that had no knowledge between good (right) and evil (left), from God’s viewpoint.
The Bible itself guide us to understand so.
In fact, in Qoeleth 10:2 we read: “A wise man’s understanding is at his right hand; but a fool’s understanding at his left.” (Jewish Publication Society). Contemporary English Version also grasps the concept inside the expression, reading: “Sensible thoughts lead you to do right; foolish thoughts lead you to do wrong”.
In other words, the Ninivites had no knowledge on the manner to decide on the basis of the God’s law. This expression (לשׂמאל ימין בין) suggests their childlike crass ignorance of divine rules.
John Gill (Exposition of the Bible, on Qoe 10:2) wrote (bold is mine): “The Targum is, ‘the heart of a wise man is to get the law, which was given by the right hand of the Lord; and the heart of a fool to get the goods of gold and silver:'’. So Jarchi, ‘his wisdom is ready to incline him (the wise man) to the right hand way for his good; but the heart of a fool to pervert him from it.'’ The ancients [Suidas in voce δεξια] used to call things wise and prudent the right hand and things foolish the left hand.”
Interestingly, In English language, too, is used the expression ‘not to know where to turn’, or, ‘which way to turn’, where the force of the metaphor (to be – undecidedly - at a crossroads, figuratively) is remained inalterated through the ages.
Contemporary English Version grasps again the concept inside the expression, reading: “In that city of Nineveh there are more than a hundred twenty thousand people who cannot tell right from wrong, and many cattle are also there. Don't you think I should be concerned about that big city?” (Jon 4:11, CEV [bold is mine])
Really, if we taking no account of God in our life, we become like people prophet Jeremiah described in his book (Jer 10:23):
“I know, Jehovah, that the way of man is not his own; it is not in a man that walketh to direct his steps.” (Darby)