On the subject of infant deaths, Joseph F. Smith shared the following,

“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.”

(Joseph F. Smith, Gospel Doctrine, pp. 455-56)