They will protect their children. Of course, please make sure that you supervise and train your children to respect and treat the dog well. In rescue, we will not place a dog with a family with small children unless the dog has been raised with them in the previous home. The swinging tail of a Mastiff can knock a small child over. If you have very small children who are just learning to walk, you may want to wait until they are older before getting a Mastiff whether it's a puppy or a rescue dog.
Mastiffs can be territorial dogs.
They will protect their yard, house, car and family from people or dogs. They want it to be known that this is their yard. They are dogs that can be very good with other dogs and with cats as long as they have had good experiences with them. This is not to say that two males cannot get along but males especially have a tendency to want to dominate each other if they have been recently neutered. An adult male Mastiff can go through 40-70 pounds of dry dog food a month.