Spirit Doesn't Change After Death

"When a person who has always been good and faithful to his God lays down his body in the dust, his spirit will remain the same in the spirit world. It is not the body that has control over the spirit, but it is the spirit that controls the body. When the spirit leaves the body, the body because lifeless. The spirit has not changed one single particle of itself by leaving the body." - Heber C. Kimball

Spirit Form of Babies Who Die

Balloons in the Sky image courtesy creative commons license flickr.com by josephpetpickle 2008

“When a baby dies, it goes back into the spirit world, and the spirit assumes its natural form as an adult, for we were all adults before we were born.

When a child is raised in the resurrection, the spirit will enter the body and the body will be the same size as it was when the child died. It will then grow after the resurrection to full maturity to conform to the size of the spirit.

If parents are righteous, they will have their children after the resurrection” 

Joseph Fielding Smith

 “Selections From Doctrines of Salvation,” p. 547