I lived with aboriginal tribes for several years after I left the Witnesses. I also got very involved in magic (not the rabbit out of the hat stuff, real magic). I have seen things from the supernatural world that I know are real. There are malevolent spirit beings. No one knows what they really are, I do not think they are the demons of the Bible. I also do not think that they are former humans (ghosts). What I do know about them is that anyone who ever comes into contact with them is TERRIFIED OUT OF THEIR MIND and their is no doubt in their mind what happened. Having said that, they won't bother you if you don't go looking for them. Contacting them takes being very involved with extreme magical practices or being very involved with someone who is a practicer of magic. These malevolent spirit creatures don't just randomly come mess with people, there has to be a reason and someone very powerful has to be directly involved. They don't transfer their "spirits" to objects, so you can't just lay something around and "infect" someone else with it.
I don't know what happened to your child. I do know that it was not "demons".
When I was a teenager (and a JW) I had several experience that I believed to be "demons". Now I know that they were not, now I think that I was in that half awake, half dreamtime state where the mind hallucinates. Also, the JW cult pushes belief in demons so hard that I think my mind was playing tricks on me. I'm not saying that is what is happening in your child's case. I'm just saying that belief can have manifestation (which is not real) if believed strongly enough.
Anyway, I think the energy comments might be closer to the truth. But, I wouldn't worry about "demons" the malevolent spirit creatures don't work that way. Unless you lately joined the Santeria cult, or became enemies of a high priestess of Kali. No, I thought not.