I agree entirely with both Chopitychop and Grumpypants. I think a lot of people are thinking about this as though the bride and groom have deliberately set out to make life difficult for people with children. Weddings are such a social minefield, not least because guests do seem to have a tendency these days to forget that the day is about the bride and groom, and focus on their own wishes and needs. If they don't want children there, it is entirely up to them. It doesn't make them bad friends, or mean-spirited or spiteful or selfish. It just makes them focussed on their own wants and needs for this one event in their lives. If you can't have things all your own way on your wedding day, when can you?
It would be an entirely different story if the couple then proceeded to guilt-trip the OP and her DH about one or both of them not attending, but as long as they accept the situation then I think it is unfair to assume that they are bridezilla and groomzilla.
I have been to so many weddings that have been spoiled by screaming children and, in my experience at least, people tend not to take their children out at the first whimper. They tend to hang on in there, smiling a bit wetly until it becomes truly unbearable. Pre-DS I went to two weddings where I finished up taking out friends' children because they weren't moving - they were quite happy for me to miss the ceremony though!
Since DS I have missed on wedding as it was no childen - DP went alone. I have been to one child-friendly wedding and missed the whole ceremony standing outside with a screaming baby. I have been to one strict no-children wedding where one couple turned up with their two small childrn anyway and there was nearly a riot as everyone else had had to arrange a full weekend's childcare.
It's entirely up to the guest how they deal with this - both go and make some sort of arrangements where possible, one go and one stay at home with the baby, neither go. Whatever choice is made, however, there is no reason for it not to be made graciously and with best wishes to the couple.
I am also slightly surprised about how many people think it is unacceptable for a father to leave his 3/4 month old baby for a couple of nights. I was first alone overnight with our baby at 3 weeks old due to DP's business trip. Neither I nor our DS exploded.