It was a warning, no contact snap, but the boy is only three and my dog is a Rottweiler so is pretty strong. I've had her a couple of months (rescue dog but hadn't been abused, taken in due to allergies of owner's new stepkids) and she's never shown even a hint of aggression (except when another dog tried to bite her ankles, and another warning snap at an adult who had only just met her and handled her roughly).
The boy (my cousin's son) is fine but obviously cried, and I'm shaken. My dog is usually so well-behaved - I'm sure she was just startled because the boy went over to her and stood directly in front, very close, at eye level and I think she felt threatened.. I think I'll have to give her a muzzle when around small children... What do you think?