I have a German/Australian Shepherd mix and she is very protective of our family (not violently) she will bark if the kids have friends over and their friends run around or want to play physically with them she will also step between them as if she is trying to separate them or herd them even sometimes. She also does this with my husband and I if we play around.

Is this normal behavior for this breed of dog or is it simply my dog?

Shepards are well known for their protective genetic makeup, they also LOVE children. She might be wanting in on the action, as they love to play, and have high energy. Here is a link you can click for information about shepards. http://www.dogster.com/breeds/australian_shepherd

All dogs are different and will have their own personalities. While certain dogs are predisposed to certain traits, that does not mean they will necessarily act that way. I would say that what you described does sound like a normal behavior for a dog that was conditioned to think that behavior was ok - if you want your dog to be a guard dog, and are training it to do so, that is how it will act. As long as the dog is not aggressive toward people there is nothing to worry about.