It is the bride and groom's day, they are spending so much money on it, and so many have the unrealistic plans for it being 'perfect'. Numbers are limited by cost and venue. I can understand the reasoning, and it isn't usually about the children' behaviour. For a family of four, if you invite the children, then that is two of your friends that you can't invite, and if you are honest, which would you rather have at the wedding, friends that make you laugh and who will continue to be part of your lives, or someone's children who may well forget the day anyway? OOH, of course it can be a bit of a bother to organise the childcare so that just the two can go, but that isn't the bride and groom's problem, it's part and parcel of having children! 
I love to see prettily dressed children running around at weddings, but only if the bride and groom genuinely want them there.