For our wedding we had no children.... but that did not include the 2 BF babies that were under 6 months. I was happy for them to be there and both their mothers spoke to me to explain the situation. Most of the time you wouldn't have even known they were there. It is different to a squirmy mobile 2 year old.

I also had to take my DS to a wedding when he was just a month old. Just something I had to do - he was exclusively BF so he had to come. I spoke to the bride and she was fine. There were 2 other babies at the ceremony, one 4 weeks older and 1 a week younger and both were palmed off for the night. The mother with the 2 month old had to express a couple of times during the night which made her miss out on some of the party and the parents with the 3 week old had dinner and left. They didn't even consider that their babies could just sleep in the pram in the corner.

So I think you should talk to the bride or your cousin and just explain that your baby is not going to need food, or a seat at the table and he isn't at the age where he will run around interrupting everyone having a good time. If they don't accept it as a reasonable request then I would consider not going. It isn't fair of them to expect you to leave DS with a stranger for babysitting and it isn't fair to expect you to travel all that way to not go to the reception because they don't want your child there. But it is their day, and if they really don't want your DS there, then I guess you have to respect their wishes.