CPS interviewed my children I have some questions!?

Okay so my sister is in the beginning stages of her divorce and she is living with us for the time being, she has an open case on her ex husband so naturally CPS had to visit my home and see where the child is living and what not. Then today at their follow up visit they told me they wanted to speak with my daughter ALONE?! Why would they need to if I'm not even involved in the situation? Other than giving her a place to stay until she gets on her feet. My daughter is only 3 years old! Her ex is very vindictive and I wonder if he is trying to discredit the people who are supporting her? But wouldn't CPS have to tell me if he is trying something like that? He has a lot of mental problems :( Hopefully someone can help to answer my questions