I have always been uncomfortable about expressing my opinion regarding young children as guests to a Bris. I suggest but do not insist that young children not attend. I have been afraid of being accused of not liking young children... which is absolutely not true. I do however believe that a Bris is not an appropriate venue for children under the age of four. First, they have a limited attention span, nowhere near the range of 30 minutes necessary for the duration of the ceremony. Second, when they begin to fidget, play, speak, fall and sometimes cry, my concentration as a "surgeon" is affected. Third, it is probably not a good idea to have the newborn baby undergoing the ceremony exposed so early in its life to so many young children.
Most parents would not think of bringing their 18-month-old child to a wedding, Bar Mitzvah or funeral. I am equally sure they would never allow their children in the operating room while another member of their family was undergoing surgery. They would want the surgeon to be able to concentrate no matter how fast the procedure. Finally the dynamic of the ceremony is completely changed when there are several young children in the room. When a guest child speaks out or cries, all the guests turn their heads and focus their attention on the speaking or crying child and not on the family and honored baby where it belongs. As a result, the intimacy, spirituality and powerful engagement of the ceremony is disrupted.
If there were several youngsters in the group, perhaps having someone watch over them in another room while the ceremony is conducted would be a good option. That way they could join everyone once the Bris was concluded.