Please take your child to a professional. I do not think your GP can help you in this case. Check what options you have in your local children hospital. Call them and make an appointment ( most of them have different unit for child s abuse). They have people trained to talk to child like yours, very young kids.
Please also find someone you can talk to, you and your DH need professional help, for you guys to face this very difficult time as well as for help your DD.
Let the professionals can make the decision on whether to report or not. If it they think it should be reported, they will do it. Do not feel guilty about it. Take care of yourself.