Ladies/moms please help

My step daughter has lived with her father and I for the past year and two months . Well her grandma drops her off right when I get home . Well today I got home about 5 mins past . She has her key well anyways I’m knocking on the door and she’s not answering so I unlock the door and she comes running out of our room. She’s panting , I ask her “ why were you in my room” . She says I was going to clean your room. I said no not in my Room I have things in there you don’t need to see. She started crying. I was like what’s wrong . What happened. ? She said “ you’re going to be so mad at me “ I had a feeling I knew what it was . So she finally confessed she found my toy . I asked her what she thought it was and she told me . She said she has been using it for a few weeks. She explained that she had been using it down there. She starts crying, before I couldn’t say anything else I was shocked . It took me a minute but I told her I wasn’t mad , shes going thru body changes and she’s exploring herself . The PROBLEM IS shes 11 going on 12 in December! I did promise her I would keep her secret but her dad and I very close and I have a guilty conscience that’s in keeping a secret from him. He’s very understanding and I don’t think he would ever say anything but what do I do ? Do I tell him or do I keep it from him? It’s a personal girl thing for her but I don’t know what to do ! Please help