Where did she find her dad? In her house, backyard, woods? Does the officer believe that the girl found her father close in time to when he was murdered?
Nine years old is pretty old. They can articulate pretty well. And actually, depending on the jurisdiction, I think the average age that is considered, “presumed unreliable,” is 7yo. And even then, a child that young can testify. The judge will usually ask them questions to determine whether they know the difference between a lie and the truth, etc… One of the most intense moments of my career was cross-examining a 6yo molestation victim. The kid was an excellent witness.
The police would absolutely question a 9yo, to get as much information as possible. They would probably have the child close her eyes, and think back to finding her dad. What does she see, what does she smell, what does she hear, is anything out of place, are the windows opened or closed, did she touch anything, how quickly until she called the police… lots of, “you are strong and smart, we need your help, we need you to tell us everything you can remember.” Also about her dad… did he seem ok the last couple of days? Was he upset or agitated? Was there anyone around other than mom or dad the past week?
They would definitely bring her somewhere she felt comfortable talking. Maybe to her bedroom. If she is at the police station, they would definitely bring her something to eat and drink, give her a stuffed animal (maybe her favorite taken from her house). And don’t forget the CPS social worker. They will absolutely be there.