AIBU to ask to bring my baby to a wedding?

You’re not being unreasonable, you weren’t rude and made it clear you were happy with either option.
Unfortunately when you have a very young breastfed baby that needs feeding every couple of hours then you can’t just leave them.
You gave her the choice so I don’t really know why she’d be unhappy, disappointed maybe, but these things happen. A couple of my close friends missed my wedding because it happened to be very close to when they gave birth and they couldn’t travel. It was a shame but obviously I didn’t blame them, we just caught up later to have a glass of champagne and share photos (both wedding and baby!)

Yeah I agree with @YouveGotAFastCar , putting in the bit about 'we can't come if you say no' in the first message was a mistake. You should have let her answer yes or no to the baby, and then replied to apologise, you cannot make it etc.

I think she's made it clear that it's no babies/children by not inviting them, though.

And while I've got no strong feelings against messaging her to confirm there's no leeway for babes in arms, your message reads a bit loaded. You've clearly tried hard to make it sound casual but give her an opportunity to say no but it doesn't quite read right.

If she hasn't replied, I think you've got your answer, to be honest. I'd make the decision about the hen do accordingly.

If DH is coming to the hen do to have the baby elsewhere and then prompt you when it's time to feed; do you think they expected that precedent to be true here too?

We had a child-free wedding purely because of the Covid number restrictions; but did allow babes in arms. That did mean some parents messaged us and asked if an exception could be made for X, Y or Z; though, for various reasons, and in the end, there were 10 or so children there between 4 days and 8 years old. We would have been in an awkward position if the Covid numbers hadn't changed, though; as whoever is told they can't have their exception is going to be pretty pissed off that someone else did... and there's no accounting for what people consider important. I can't leave my breastfed 1.5 year old overnight yet - I'd never ask to bring him to a wedding for that reason but I can imagine some people would! Is the toddler more or less important than your 4-month-old? etc.

Hmm I don't think you were totally BU, I got married recently (child-free wedding) but my pregnant friend went early so had a 2 week old on the wedding day. She sent a similar message (as if I'd have said no!) and it was fine, they all came, but to play devil's advocate, if this is your oldest friend you should probably have had the discussion in person. Also your baby is 4 months old so you knew this was coming (you must have had an idea it would be child-free if she's an old friend?). Also our general rule of thumb was babies up to 6 months would have been fine, but beyond that we'd have preferred them not to come. Maybe that's the position she's in.

Anyway, if you're not invited to the actual ceremony, it seems too much hassle to go tbh.

Well she did say no children because their names were not on the invite it's you that has caused "confusion " there is no confusion.

She specifically said no children it was really up to you to go or not, I don't blame her for not replying she's probably stressed and your request is maybe her feel awkward, just don't go if you can't leave the baby.