Jet2 let us book and said we just call up after baby arrives to update her name and passport details
You have to add them on for the flights as you need to insert DOB. I think you can wait to insert passport details when you check in but this might vary on airline. In terms of accommodation, transfers etc you should be able to include them now and request cot etc.
Worth checking with the specific airline, but we had to book flights last week and were advised to make up the name and DOB of the baby and then call up to change it once we have the actual details.
We booked a holiday for summer and I was told to book our seats and to add baby on after they've been born, they can't add a baby without a name and DOB
Not a travel agent but usually airlines allow you to add a baby on as I’m pretty sure they travel free or for very little when sitting on your lap