I never put helmets on my kids when they were in trailers. No cops ever looked in, and if they had, i would have either talked my way out of it, or paid the fine. You could bring the helmet along and demonstrate to the cops.
Also, my kids generally responded well to the appeal to a higher authority. If you there was a sign, i would tell them that the sign said whatever I wanted them to do, I would enlist anyone in a uniform to tell them the rules I wanted them to follow and they would usually fall into line.