There are many breeds that would fit this description. One of the most known for these reason is the Bernese Mountain Dog. They're known to be calm natured, however loyal, strong & intelligent dogs. They're fantastic for kids as they aren't a very aggressive breed, love to play sports and spend time with humans whenever possible. In terms of a guard dog, the breed's large size and loud bark is effective in deterring any unwanted intruders.
Alternatively, the Boxer, may appear menacing, however they are filled with a desire for human affection, most commonly from kids. They are an active, loyal, fun filled breed, but are known to be alert to danger and use their agility and athleticism to protect their family.
The list can extend to Bullmastiff, Doberman Pinscher, German Shepherd, Great Dane & Rottweiler.