We had 220 people at our wedding and had one table of children - they were immediate family members. A couple of the younger children left early with grandparents etc.

We have a wedding in April which is child-free and it doesnt bother me. It is my closest couisn who I love dearly and they are also having a large wedding and due to additional space constraints they simply cant have everybody's kids so they are having none. Am not offended in the slightest and I certainly will not be boycotting it either. I think that guests have to realise that it is not a personal slight from the bride and groom usually it does come down to finances or space.

I have also chosen to go to weddings without children, when DD was 5 weeks old and 7 weeks old we had weddings to go to of close friends. I expressed and left her with my mum who was only 10-20 mins from the reception venue. Both times I took "time out" from the reception and went to the car to express - you could say I surprised a few smokers!

I dont think there is anything wrong with parents being honest and saying that sometimes it is nice to have an evening without their children. As much as I love my kids and they pretty much come everywhere with me sometimes it is ok to have grown up time too.

But in answer to the OP I would say that a baby becomes a kid for wedding purposes when they become mobile ie crawling as it is then that they are more likely to be for want of a different word "disruptive"