Inviting six children up to the front of the room. Put a prophet sign on each child. If you would like you could dress them up to look like that prophet.
Have the children read the scripture about what they prophesied. Have the rest of the primary children stand up when they hear the prophet prophesy of Christ's birth. Continue with each prophet, ending with Samuel the Lamanite. After him, sing the song "Samuel Tells of the Baby Jesus" with the children. You may want to do actions or show pictures to go along with the song.
Explain to the children that all of these prophets lived years before Jesus was born. That they prophesied of his birth and Jesus was indeed born.
Bear your testimony.