First and foremost, make sure your child feels safe. Then smile and make it a no-big-deal. Our world is filled with insensitive individuals who don't quite know how to interact when encountering a situation that makes them uncomfortable.
Remember, disclosing your child's BFRB is their decision, not yours. It's up to them if they want to tell anyone, let alone a stranger, about their BFRB.
Making sure your child's emotional and physical well-being is protected during uncomfortable encounters in public helps them know they can engage in community settings without fear, which in turn helps them participate more fully in community as their authentic selves.