They only invite your child as a courtesy.. You can opt out by hiring a baby sitter and going yourself and if anyone asks why you didn't bring the little tyke just tell them he has the runs and you hired a setter to care for him because this was just as important.. They will not inquire again and will appreciate your decision..
Beautiful.. We once shared our lives with one (lady o'shannon) she was the sweetest but her size alone kept potential burglers well beyond our property.
I think all children, all ages, (if they are relatives of the Bride and Groom) should be invited and I think they should all (even the ones with the runs) attend. I do not like to exclude children from all the different aspects of family gatherings, including weddings and funerals. But I do think this answer is very helpful for most people.
Thanks Njoy for the helpful vote but I was responding to the parents in that they shouldn't feel obligated to bring their children if they choose not to and the hosts shouldn't be insulted because of it. I agree, all children should be invited but not all parents should feel obligated that their children attend