Joseph teaches that children who die in mortality will be raised as children and will being to grow into adults in the resurrection.
"Discourse, 7 April 1844, as Reported by Thomas Bullock," 21, The Joseph Smith Papers website, accessed November 24, 2023
Mothers you shall have your Children for they shall have it— for their debt is paid there is no dam[natio]n awaits them for they are in the Spirits— as the Child dies so shall it rise from the de[a]d & be living in the burng. of God— it shall be the child as it was bef it died out of your arms children dwell & exercise power in the same form as they laid them down