Ugh, My SIL and her ExDH used to do this all the fricken time. She does many many many rude things though. Not entirely sure how she internalises her actions but it's very bizarre.
It is so incredibly rude to do this. And especially to not even offer 'hey, we were going to grab maccas on the way over, do you mind and would you like us to get you some too?'
All it takes is some courtesy!
Edited to add: This has nothing to do with the fact you have a newborn. It is unacceptable in ANY situation!