Rottweilers are known for being very strong, as they were once bred for herding livestock and pulling wagons. They are fiercely loyal dogs, which means that they can exhibit aggression towards strangers. Strict training can help control their dominant personalities.
The Bullmastiff is quite the large dog breed, which wouldn’t in itself be an issue but it can also be quite an aggressive dog. Since these large dogs don’t know that they’re big, they can accidentally knock over kids without even realizing.