Ghosts Appearing as Children [Latter-day Saint perspective] Elder Mark E. Petersen (1900–1984) of the Quorum of the Twelve Apostles explained that the physical body and our spirit are similar in appearance: “Every one of us is a spirit, and our spirit occupies a body of flesh and bone. The spirit is the real person. Our spirit resembles our body, or rather our body was ‘tailored’ to fit our spirit. The spirit bears the image and likeness of God, and the body, if normal, is in the image and likeness of the spirit” (The Way of the Master [1974], 124; see also 1 Nephi 11:11).


From this, the spirits of those who have dies should  look like the individual as they lived; however, it is also taught that our spirits were adults before this life began.

As I understand it, this earth is also the world which is inhabited by those who have died.  Assuming that ghosts are those spirits of those who currently do not have a body (either due to death or as one of Satan's host), why would ghosts ever appear as children?
I don't know if there is a definitive answer out there. In the gospel fundamentals manual it says:

We are the spirit children of our Father in Heaven. While we lived with Him in heaven as 
spirits, we did not have physical bodies. It was part of His plan for us to be born on this
earth and for our spirits to receive bodies of flesh and bones. Our spirits are what gives
life to our physical bodies while we live on the earth.

Just as to be born was a part of our Father in Heaven’s plan for us, another part of His
plan is for us to die. When we die, our spirits leave our bodies. Without the spirit, the
body has no life and is placed in a grave.

But our spirits lived before our birth, and they will continue to live after death. When we
die, our spirits will go to the spirit world to wait for the resurrection, while the body
remains in the grave.

In the spirit world our spirits will have the same form as when we lived on the earth with
bodies of flesh and bones. We will look as we do here. We will think the same way and
believe the same things as we did here. Those who are righteous in this life will still be
righteous. Those who were unrighteous will still be unrighteous. We will have the same
desires after we die as we had while on this earth.


From the last two paragraphs, we learn that our spirits lived before our birth, and they will continue to live after death. If our spirits grow at the same rate our bodies do or normally would (or vice versa) then a ghost/spirit could appear as a child if a person died as a child (see also Gospel Doctrines pg 453-454 starting about 3/4 down). Normally just spirits do not appear to man, but it does happen (see Doctrines of Salvation Vol 1 'The True Guardian Angel' or DC 129 or Gospel Doctrines pg 436 about 2/3 down)

Evil spirits, those who followed Satan, are here on the earth to deceive man (DC 50:2,31-32). They can appear in many forms. They appeared as an angel to Korihor (Alma 30:53) and it's probably safe to assume they can take other forms.

Side Note

To determine if a spirit is of God or the devil see DC 129