I would never insist that a wedding is child-free, but I can see the appeal.
DP and I are getting married next year, but we are ancient (nearly 50) and none of our friends have really small children (babies/preschool age). Although we are inviting friends including their children, we are kind of hoping that most of the ones whose children don't really know us will leave them behind (we may drop subtle hints along those lines...).
My DCs and nephews and nieces are all teenagers, and they will be there, as will a few other children of close friends who also know us, but I know from experience that younger children at weddings can get incredibly bored, don't like the food, are hungry at the wrong times etc etc, and the venue we have booked will not have any running-around space if the weather is bad. We will try and lay on some entertainment for children, but it will be tricky.
And as space is limited, for every potentially bored child who stays at home, we can fit in a friend who might actually want to be there, which seems like a better outcome all round. But we certainly won't be excluding any close friends because they don't have childcare.