Quite some people here declared that they are not Christian but they pray to God. What is the difference between a Christian's prayer and a non-Christian's prayer?
For those not yet received Jesus, you are just one step away from the door to salvation and a new life. You hope Jesus to protect you. But the devil will challenge what is your qualification to be protected by the blood of Jesus.
To give you no doubt that you are protected and to give devil no excuse to challenge you, I suggest you said the following prayer with a truthful mind:
something wrong with that 師母! it doesn't work. we are not in a movie. some people may open the bible (the physical book) in the room hoping drive away any evil spirit. it doesn't work. what works is the truth (God's words) in the bible. when you pray, proclaiming the name of Jesus, all evil spirit will flee. if you really are scared, you may on some worship songs CD in your home. that may help as well. the point is: if you are a worshipful christian, keep yourself holy and not to leave any ground to the devil, you will feel very peaceful. don't you know that the one inside a Christian is bigger than the one outside.
if you have read the bible, you will know we Christians will have the authority to judge Satan and all evil spirits together with Jesus when He comes back again. Actually, evils are scared of Christian because we will judge them at the end of days. Knowing this fact, does it make you feel better?
i think your worry is all from what your son saw. You "assume" that was ghost afterall. may or may not, no one knows. why make yourself bother so much? it is not necessary, really.
To tell you one more thing, kids may see ghosts and they may also see angels! my daughter used to see things on the walls or in the air. sometimes she said "uncle" or "auntie" when she saw things that we could not. At the beginning, me and my wife was a bit scrared. later, i read from some books that children may be able to see angels (you may guess why adults cannot). whether it is true, it makes us feel more comfortable. afterall, if we know that we are ok Christians, nothing is scary at all.
I suggest to seek God and try to form a closer relationship with Him. You won't be scared anymore.