Mormon, the prophet-historian, noted that Jesus taught the people for three days, and after that “he did show himself unto them oft” and did administer some kind of sacred meal often—meaning, perhaps, the sacrament.
Verse 14 describes what can happen when all of God’s people are spiritually prepared, worthy, and receptive—with no wicked distracting. All of this will happen again at the Second Coming, when telestial distractions are removed. The Lord will then reveal “many great and important things pertaining to the Kingdom of God” (Articles of Faith 1:9).
The children’s tongues were “loosed,” apparently speaking things beyond their mortal years, possibly penetrating the veil, using the language of God, and revealing “great and marvelous things” about the premortal life and even the postmortal life—things even greater than what Jesus had taught these righteous disciples.