I've been present at coming up to 400 weddings in the last 4 years, and I can quite categorically say that children can (and on a regular basis, do) disrupt weddings. There is nothing that beats a traditional wedding (ceremony, drinks, photos, sit down meal) for creating a miserable, bored, hot, tired and hungry child, and the difference between a child and an adult, is that every person in the surrounding area has to know when a child is bored/hungry/miserable. There is no reasoning with a young child in that state either - you might as well be flogging a dead horse.
The decision is whether you are accepting of children's tantrums enough to take all of the above and shrug and say 'that's kids for you', and acknowledge that in all likelihood, there will at some point be a miserable screaming child taking the limelight (usually right when you'd like a photo of them), or whether in actual fact, you're not willing to accept it.
The majority of people who have children will invite at least close family children (i.e. neices/nephews etc) to the wedding. But if you don't have children, and don't particularly feel attached to others, then there's a good chance you are not going to invite them.
It's well worth considering that not everyone is in the position of being able to splash out inviting everyone and their offspring. Children do not always levy a much cheaper rate than adults, and in fact a lot of wedding venues charge adult prices for any child over 12. This can seriously restrict the numbers of people couples can invite. Often the thought process goes something like this:
- There are lots of children in our family/friend group.
- X amount of children = X amount of money
- We can't afford to feed all the children AND all the adult
- It's unfair to invite only some of the children and not others
- We won't invite any of them, then that's fair.
The same often goes with cousins, not just kids (though cousins are much less prone to being miserable).