They will protect their family more along the lines of a watch dog than guard dog. If your intent is to have a dog that is a guard dog then you must think about another breed. They will often bark and let intruders know they are not accepted. Once you accept the guest, chances are good that they will too. Their mere presence and bark will scare the bravest of burglars.
Mastiffs are wonderful dogs with children.
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.