Unless my child was a pathological liar or had some other psychiatric or mental health issues, I would believe my child.
My own daughter always struggled with lying. I believe it was (is) a spirit because sometimes it seems she really can't even help it. That girl will lie about anything, everything, something and absolutely nothing. When she started developing, I stopped letting her stay overnight at my SIL's house because my nephew was getting kinda wild and my BIL is unsaved and kinda questionable. My biggest fear was that one day she would accuse someone of something and I wouldn't know whether to believe her. To avoid that, I stopped letting her go there.
But anyway, you have to know the child and react accordingly. If the child isn't a liar or otherwise problematic, I would believe the child and let the police sort it out. Even if she's lying, it's better to err on the side of caution.