OP, you do realize that wedding ceremonies can last over an hour? And it’s not about sitting still. It’s not exactly quality time with the cousins as you sit there quietly. So I’m not sure why you think you’re nine-year-old is going to miss oh so much quality time with his teenage cousins DURING A CEREMONY WHERE THERE IS NO TALKING
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I am Catholic. When someone in my family marries another Catholic, they usually have a nuptial mass. These tend to range from 45 minutes to an hour and a half. It's hard for kids--and especially hard for kids who don't regularly attend Catholic services. They start asking questions--they want to know about the vestments--which are unlike any clothes they've seen; why people sit, stand and kneel; what the big words in some of the Scripture readings mean, etc. They are not misbehaving, but their whispered questions and answers become very annoying to the other people around them.
And...it is unlikely you actually know the ages of the other kids who will be in the room. Maybe a friend in her 20s has an 8 year old stepson, for example. You haven't seen the guest list; you are assuming.
You're looking for an excuse--admit it.