It's a variation of the time-honoured aphorism 'Children should be seen and not heard.'
There's definitely "not (be) heard" omitted here. Is this a mistake or an acceptable expression?
pp184)King Edward loved his grandchildren, but he treated them very strictly. They were not allowed to speak at the table. One day, one of his grandsons called, "Grandpa!" when King Edward was having lunch with the King of Sweden...The little boy tried again to get King Edward's attention. The King then yelled, "Little boys should be seen but not heard at the table."
The "be" distributes to both "seen" and "heard."