As for smoking, I'd not say no to her being there unless she was in a confined area with smoking, or people were smoking around the kids without a care for their health. Considerate smokers bugger off to a section of yard nobody else is occupying. If the smokers weren't considerate, we'd be going home. I'd be explaining to my daughter why. I don't lie to her often though... I mostly give her age-appropriate explanations. I see no point in fibbing.
If she was too young to understand, I'd just pull the, "I'm the mum and therefore the boss," thing.