A Snapping Bichon

Q: I'm just starting to look into how I can solve this problem. I have a 9-month-old bichon frise, who has given us nothing but joy. Just lately she has begun to snap at children occasionally. We cannot identify a common denominator in any of the instances. She has never snapped at an adult. It has happened a couple of times with children under 5, and twice with children around 10-11 years of age. Her breeder was an older woman, and we got her when she was 6 weeks of age. She has been checked by the vet and is healthy. Other then these few times she is great. I was wondering is this common and is there anything you could recommend as she is the first dog we have ever owned. No one is aggressive with her; if anything she is the queen of the house.