And now, O Jehovah my God, Thou hast caused thy servant to reign instead of David my father; and I am a little child, I do not know to go out and to come in;

And now, O Lord my God, you have made your servant king in the place of David my father; and I am only a young boy, with no knowledge of how to go out or come in.

And now, O Lord God, you have caused your servant to reign in place of David, my father. But I am a small child, and I am ignorant of my entrance and departure.

See 1-Kings 2:2 note, and on the hyperbole contained in the phrase "little child," compare Genesis 43:8; Exodus 33:11.
How to go out or come in - This expression is proverbial for the active conduct of affairs. (See the marginal reference.)

I know not how to go out or come in - I am just like an infant learning to walk alone, and can neither go out nor come in without help.

And now, O LORD my God, thou hast made thy servant king instead of David my father: and I [am but] a little child: I know not [how] to (f) go out or come in.
(f) That is, to behave himself in executing this charge of ruling.

And now, O Lord my God, thou hast made thy servant king instead of David my father,.... Removed by death, in whose stead he reigned by the appointment of God, and through his overruling providence, notwithstanding the attempts made to prevent it, and therefore to God he ascribes it:
and I am but a little child; not in age and stature, but in knowledge and understanding; for though his father called him a wise man, and he was judged so by others, and really was one, yet in his own opinion and thought of himself such was his modesty and humility, that he was but a child as to his intellectual powers and capacity for government: some understand this of age; and the Jews commonly say he was but twelve years of age when he was anointed king, which they reckon thus; that he was born at the time that Ammon ravished Tamar, two years after which was Absalom's sheep shearing, when he slew Amnon, on which he fled to Geshur, and was there three years; here are five years; he returned thence and was at Jerusalem two years; lo, seven years; he rebelled and was slain, and after that there was a famine of three years, which make ten; and in the year following David numbered the people, which was nine or ten months in doing; the next year he died, which was the fortieth of his reign, in all twelve years; so reckon Jarchi and Kimchi; and Eupolemus, an Heathen writer (n), is express for it, who says, that David, when he had reigned forty years, delivered up the kingdom to Solomon his son, being then twelve years of age, which he must receive from the tradition of the Jews; the same is said by several of the ancient fathers, as Ignatius (o) and Jerom (p); but this cannot be fact; for, if so, his son Rehoboam must be born to him when he was but eleven years of age; See Gill on 1-Kings 3:5; it is best therefore to interpret this of the sense he had of the weakness of his understanding, and of his incapacity for government, as the next clause explains it:
I know not how to go out or come in; in the administration of government, to execute his office as a king, in allusion to shepherds, as kings are sometimes called, going in and out before their sheep.
(n) Apud Euseb. ut supra. (Praeparat. Evangel. l. 9. c. 30, 31, 32.) (o) Epist. ad Magnesios, p. 141. Ed. Voss. (p) Epist. Rufino & Vitali, fol. 24, 25. tom. 3.

I am but a little child--not in age, for he had reached manhood (1-Kings 2:9) and must have been at least twenty years old; but he was raw and inexperienced in matters of government.

Child - So he was in years: not above twenty years old; and withal (which he principally intends) he was raw and unexperienced, as a child, in state affairs. Go out, &c. - To govern my people, and manage affairs.