On top of -not in place of- the advice given, I wonder with the other poster who mentioned it if maybe your child could be suffering from scrupulosity.
I know a 5 year old who is being raised in a devout Catholic family. His many brothers and sisters have no problems and are balanced in the area of sin. This little boy can’t play for ten minutes without stopping to say an act of contrition. We’re not talking little boy play, either. He is mortified that he commited a mortal sin when he does something he thinks he shouldn’t have and practically paralyzed if he can’t get the act of contrition out immediately. His family thinks it is great that he recognizes the effects of sin at such a young age and they giggle to see him stop in prayer. I hurt for the little boy who has a distorted view of God and of sin and whose faith paralyzes him with fear constantly.
With how religious you are, I wonder if your praying over your daughter and the other externals are actually making her feel worse instead of better. If she is struggling with very normal thoughts but labeling them as evil and therefore from the devil, your praying over her and putting holy water on her might be reinforcing her labels and fears.
In conjunction with seeking medical advice, I would recommend a radically new response to her. By all means, pray for her in private. I wouldn’t make a show of praying over her, though. While seeking medical advice, I would talk to your priest about limiting her daily prayer time. If you do that, I would explain that we all have temptations and this is a normal part of life and that’s why we pray in the morning for our guardian angel to help guide us and for St. Michael to protect us. That it isn’t a sin to have tempting thoughts and Jesus knows we don’t want them and if it helps we can say a short prayer like “Jesus, Mary, and Joseph” to help us to choose to do the right thing.
Notice I didn’t say to fight the devil or to fight the bad thoughts. She’s so caught up trying to fight these thoughts -whatever their source- and probably needs to hear that she doesn’t need to fight them and therefore dwell on them, but she needs permission to go on in life because what God cares about is what she does about it, not what comes into her head.
I would respond to her as if she had just said her favorite number is 7 or she named the plant Steve or she had trouble tying her shoes when she says the devil spoke to her again. I wouldn’t let on that I was taking note of what she said to tell the doctor and was really quite concerned, but would work to distract her immediately by changing place and activity to something active and fun.
What is it she says she is hearing? I’m trying to imagine how the conversation goes.
Daughter: I can’t get him to stop telling me to do bad things!
Mom: What kind of bad things?
D: He told me to hit my brother after he took my toy.
M: Oh! You were probably frustrated and angry that your brother took your toy. You know, I get angry when people take what is mine, too. Sometimes I think about doing mean things to get back at them. It can be hard sometimes to not lash out in anger when we are hurt. You did a really good job in recognizing your anger and not acting on it! I’m proud of you for using your words instead of your hands!
Or is your response more like this?
Mom: Jesus, I ask you to send down your Holy Spirit and to cleanse my daughter of every sin and impurity. I ask you to cast out the demons which torment her and to help her mind to be clear of these demonic thoughts. Jesus, we trust in you. Do you feel better now?
I would still bless her room and include prayers to St. Michael at the end of the day or whatever prayer or sacramental helped her to feel safe -again, limited to one or two after discussion with the priest-, but I would do it explaining to her that all people are tempted, even the saints, so this is how the church helps us to deal with the tempting thoughts instead of giving in to them. Then she knows the thoughts aren’t the bad part, but what she does is the important thing. And she knows that she isn’t the only person this happens to. It helps to put those temptations into perspective. To a six year old, the world can be a scary place and the idea of sin and hell can be terrifying. You could share with her some of your tempting thoughts that you decided not to act on and how proud of you Jesus was when you didn’t say that mean word or throw that shirt after the tempting thought came into your head to do it. She has an overworked sense of God’s justice, so she needs to hear a lot about God’s love and mercy whatever the cause of her thoughts.
If you cut out your own actions which might be adding to or fueling her fears, you and the doctor will have a better idea of what is going on inside her and how to address it. A lot of psychiatrists will medicate her at her age, so I would recommend doing your research and talking to your pediatrician first. After discussing what the thoughts are telling your daughter to do and her ability so far to resist the directions and anything you or others are doing which could be playing a part in the situation, you can discuss with the pediatrician whether she thinks a psychiatrist, psychologist, or a therapist would be the best place to start. You could then discuss with the chosen mental health worker what steps you could be taking to help your daughter.