Judging by the Catena Aurea, the consensus among the Church Fathers was that "the childlike" referred to in Matthew 11:25 are those who are humble:
Augustine: That the wise and understanding are to be taken as the proud, Himself opens to us when He says, "and hast revealed them unto babes;" for who are "babes" but the humble? Gregory: He says not "to the foolish," but to babes, showing that He condemns pride, not understanding. Chrysostom: Or when He says, "the wise," He does not speak of true wisdom, but of that which the Scribes and Pharisees seemed to have by their speech. Wherefore he said not, "and has revealed them to the foolish," but "to babes," that is, uneducated or simple; teaching us in all things to keep ourselves from pride and to seek humility. Hilary: The hidden things of heavenly words and their power are hid from the wise, and revealed to the babes; babes, that is, in malice, not in understanding; hid from the wise because of their presumption of their own wisdom, not because of their wisdom... they who disdain to be made babes in God should become fools in their own wisdom.... Chrysostom: And wherefore were they hid from them? Hear Paul speaking, "Seeking to set up their own righteousness, they were not subject to the righteousness of God."
The Greek word is an inflected form of ne^pios; it is used when Jesus quotes Psalm 8, "Out of the mouths of infants and nurslings you have brought forth praise." St. Paul uses the word when referring to a "teacher of the simple," as well as to "infants in Christ," and repeatedly in the famous verse, "When I was a child, I used to talk as a child, think as a child, reason as a child; when I became a man, I put aside childish things." (For completeness, he uses it in theseversesaswell, and Hebrews 5:13 says that the child "lacks experience of the word of righteousness.")
In the Epistles, childhood is primarily a matter of immaturity, of lacking what an adult disciple ought to possess. This is not to suggest an incompatibility between the Epistles and the Gospels; it is to suggest that if we want to understand why the thought that God has revealed things to the childlike that were hidden from the wise, we shouldn't go with the simple notion that childhood=good, adulthood=bad.
And, as always, whenever I write something like "the Greek word used in that verse," it's based entirely on what I find by poking about this site. I myself have less Greeke than a plate of lutefisk.