ANSWER: Ghosts are one of the many signature symbols of Halloween, along with skeletons, carved pumpkins (jack-o-lanterns), and a general preoccupation with pain and death. Interestingly, the Bible does have something to say about ghosts in general and how they are related to Jesus' ministry.
The Old Testament of the KJV Bible refers to ghosts eleven times while the New Testament refers to them one hundred and thirty-five times! Most New Testament references are derived from the Greek word pneuma (Strong's Concordance #G4151) which roughly means "spirit." The KJV errorneously refers to God's spirit as the "holy ghost" (instead of the more correct "holy spirit") one hundred and thirty-five times.
Surprisingly, one reference to ghosts involves Jesus' disciples using the word to describe him! This event occurred when Christ sent his disciples, in a boat, to the other side of a lake they were near. Sometime in the middle of the night, after he was done praying, he noticed the disciples' boat was being tossed by the waves. He then proceeded to perform the incredible miracle of walking on water toward the ship! When the disciples saw him on the water the screamed in terror "It's a ghost!" (Matthew 14:22 - 26).
Your Email suggests that what you may be referencing, in regard to what your children sometimes see at night, may be evil spirits (demons). To determine if demons are involved, we suggest you first do some thorough investigating. Many times, children of a young age have imaginary friends. It is their process of maturing to become sociable beings. When they become older, they outgrow their imaginary friends. Make sure your child does not really see an aparition.
If, after all your investigation, the "ghosts" proves to be real, then demons are likely. Christ, in Matthew 12, discussed a situation where a man had an unclean spirit (demon) leave him for a while. When the evil spirit could not find any place to go, he came back and found his "home" (the person he had possessed) "empty and clean." This means the person did not fill the void left by the spirit with something that would keep him out (e.g. God's spirit). Not only did the demon possess the man again, but seven of his evil friends came with him! This illustrates how serious a problem such "ghosts" can be.
In Matthew 17, the disciples came to Jesus saying they could not cast out demons ("ghosts") from a boy who would throw himself into fire and water (Matthew 17:14 - 21). This was because they lacked a sufficient amount of faith in God. Jesus stated that the possession was so severe that the unwanted guests could only be forced to leave by fasting and prayer.
If you discover you have such an unwanted spirit hanging around, find at least one or two others who are mature Christians (preferably with more faith than you possess). Get together with them and pray, in Jesus' name, that God remove the demon and have it never return. We, of our own power, cannot command demons (ghosts) to do anything. Jesus Christ, however, through his life and sacrifice, overcame all adversaries and nullified the works of the devil. His power can free you.