Hi, I'd like to help you with your question.
It sounds like there is something going on in your ex's home while the kids are there that is making the kids uncomfortable. And your daughter might be being told things about you or the situation between you and your ex that make her feel upset.
Does your ex have a history of abuse, either physically, emotionally or both? If he is mean and controlling, then he is most likely being this way to the kids. Your daughter may not be able to tell you and fears the repercussions if she does. So she distances herself. Your son may either not have the same threats against him or he does not feel the same pressure his sister does.
You should pursue whatever is going on. Can you visit with the kids and take them out? If so, you should. In order to help your daughter talk to you, you should set up a way to reassure her she is safe. Or she could write you a letter about how she feels and send it. The idea is to find some way for her to communicate with you that she feels safe and open to.
If you feel there might be something going on with your ex being abusive, contact your attorney about your concerns. Your attorney may not have any recourse now without proof, but you could find out what your options are and how you can handle this situation legally.
Let me know if I can help any further,