Dog getting protective of kids?

We have an australian Shepherd and she has been good until a few months ago. We live on a farm and share property with my husbands brother so there is always people coming and going. She always would bark at people when they came, even his brother if he left and came back. She has always been very careful of who she approaches and usually will only go up to people if me or my husband are talking to them. Then she is their best friend. However she has recently gotten protective of our 2 year old daughter. My husbands niece who is 14 was playing with my daughter with a bubble wand and the dog nipped at her because she felt she was going to hit our daughter. Then the other day our daughter and 4 month old son were in the stroller and my husband stepped away for a second and a man working on the barn came over to say hi and she nipped at him. She only does this when we are not around so it is hard to correct. She has growled at people that approach them, we are around but not right there next to the kids when this happens. How do we correct it? We will be moving soon and have our own place so there will not be as many people but I still want her to know it is not okay to act that way.