If they are both socialized to each other early on, children and Shelties can become great friends. Any time a dog is around a child, they should be supervised, of course. Shelties can sometimes attempt to "herd" children, for instance. But, overall, they make wonderful family pets and are very loyal and loving.
There are several statements about Shelties being "Better In Bunches" or "Like Potato Chips - You Can't Have Just One". Each Sheltie, of course, has their own attitudes and personalities, but the majority of them do well with other Shelties, or other breeds. Many say, though, it is best to have both genders and not just males (or just females). We, however, have been very lucky with very very few "tiffs" and never any serious aggression, despite owning both genders in the same household.
We are very proud of our dogs temperaments of being mostly calm, quiet, and social.