What would you do in this situation?

Mastiffs are GREAT dogs!! Call the childs mother and talk to her. Tell her that you understand she has a fear of the dog and if she would like, she is more than welcome to come meet the dog without her daughter there to ensure the dog is "safe". Also, tell her that you would love to have her daughter over again and if you have to lock the dog up while the daughter visits, you would be more than happy too!