Joseph Smith teaches about Children in the Resurrection

The spirits of our children are immortal before they come to us, and their spirits, after bodily death, are like they were before they came. They are as they would have appeared if they had lived in the flesh, to grow to maturity, or to develop their physical bodies to the full stature of their spirits. If you see one of your children that has passed away it may appear to you in the form in which you would recognize it, the form of childhood; but if it came to you as a messenger bearing some important truth, it would perhaps come as the spirit of Bishop Edward Hunter's son (who died when a little child) came to him, in the stature of full-grown manhood, and revealed himself to his father, and said: "I am your son."

Bishop Hunter did not understand it. He went to my father and said: "Hyrum, what does that mean? I buried my son when he was only a little boy, but he has come to me as a full-grown man -- a noble, glorious, young man, and declared himself my son. What does it mean?"

Father (Hyrum Smith, the Patriarch) told him that the Spirit of Jesus Christ was full-grown before he was born into the world; and so our children were full-grown and possessed their full stature in the spirit, before they entered mortality, the same stature that they will possess after they have passed away from mortality, and as they will also appear after the resurrection, when they shall have completed their mission.

Joseph Smith taught the doctrine that the infant child that was laid away in death would come up in the resurrection as a child; and, pointing to the mother of a lifeless child, he said to her: "You will have the joy, the pleasure, and satisfaction of nurturing this child, after its resurrection, until it reaches the full stature of its spirit." There is restitution, there is growth, there is development, after the resurrection from death. I love this truth. It speaks volumes of happiness, of joy and gratitude to my soul. Thank the Lord he has revealed these principles to us.