Kids don't belong everywhere, but there are some places where their presence is marginal. Kids aren't usually allowed at a fancy restaurant, but how fancy is too fancy for someone under 13? Kids aren't allowed in most bars, but parents can and do bring their babies to some breweries. The world isn't built to accommodate kids most of the time, and that's a sad reality families have to face.
Weddings are one of those events where kids may or may not be allowed. On one hand, it's a loud party with drinking that goes on until very late. Having kids present could be a bit of a liability. On the other hand, it's nice to celebrate marriage with the entire family, especially if parents make up a great deal of your wedding attendees. Whether you have a child-free wedding is your choice, but there is an appropriate and inappropriate way to communicate that you don't want kids to come. This millennial couple did the latter.