I think it can also depend on what stage of your life you get married in relation to your friends and family. If you are one of the first, then chances are there won't be lots of kids to invite.
If you get married later, when lots of your friends and family have had kids, you can suddenly be looking at almost doubling the numbers of people that you want to attend.
And with the best will in the world, it isn't always possible to find a big enough venue that you like at a reasonable cost etc etc and compromises have to be made. If you have fixed numbers and it is a choice between inviting all your friends and no children or half your friends and their children and leaving out half your friends then what would you do? Whatever you chose you can't please everybody (or necessarily yourself) but that is sometimes the nature of compromise.
I have a friend who, because of limited numbers at the place that she was holding the reception, had made the decision not to invite children (also because it was a late afternoon wedding and an evening reception). She had made an effort and sorted out a baby sitting sort of service for friends that had to travel with kids but it wasn't at the place the reception was being held - and they could come to the church if they wanted to, plus tiny babies could come to the wedding.
Despite being told this, her MIL-to be invited some young family children that had not been invited , delayed telling friend until after plane tickets and all sorts of arrangements had been made. Led to a big argument with friend who then had to un-invite some of her friends that had already been invited... Got her off to a bad start with her MIL and put her in a really difficult situation with some friends.