Glen Terriers are excellent family dogs and can be kind and playful around kids. However, they have strong, muscular bodies (not to mention they can be rough playmates), which can be too much for a small, young kid.
As with all dogs, teaching children the proper way of approaching and touching dogs is a must. Regardless of the dog's friendliness, dogs and kids should never be left with each other without an adult to supervise them. Otherwise, the two may offend and injure each other. In addition, discourage children from messing up with a dog's sleep and food.
When exposed to other pups since puppyhood, Glen Terriers can do well with their fellow dogs. However, their strong personality is bound to be in quarrels. Also, their history in hunting small prey makes them unsuitable for living with free-roaming small pets (cats, gerbils, mice, rabbits, hamsters, etc.), as your Glen will likely chase and kill them.