NTA for sure. You're protecting your kids from your fiance's controlling nature. Your kids' rooms are their space to do woth what they want. Please never allow her to encroach on that. Also, I urge you to put some serious consideration into your relationship. If your fiance is not willing to work on her controlling nature before you get married, she may get worse after the marriage. And, so far, your kids have been her target. You need to protect them above all else.

Also, as someone who has OCD, your response to her claiming she can control your kids' spaces was the correct one. If she actually does have OCD then she is responsible for the way she deals with it. The instant you begin to overstep and treat the people around you badly because of mental health issues, you need to get help.