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