I am on the don't let her go bandwagon. For two reasons, first: if the aunt asked her to keep the dog a secret it's most likely because she knows she isn't/won't be allowed to bring the dog, and a Rottweiler is reason enough to not introduce a new animal, second: and on a more personal level, when I was six, it was my birthday actually, my mother specifically asked her cousin to not bring his normally well behaved dog to my party because we had two dogs of our own and she was concerned about how they would get along. They didn't and I almost lost an eye when the cousins dog, a Great Dane and normally a docile breed, grabbed me by my face. Needless to say my party was cancelled. I spent the day in the hospital got stitches in my eyelid, still have a scar almost 40 years later and I am terrified of Great Danes. You never ever know what will happen when two strange dogs meet.
So IMO... No no way, no how, would I let my daughter be a part of that.