I am sorry to say this but there are only a limited amount of bulkhead seats on a plane, airplanes can and do change and you really can't put the blame on an airline for minor operational issues like this. I understand it will be very hard for you and your child, but I really don't see a big issue here.
I am sure there are people with multiple kids, pregnant people and people at similar situations like you who will not be seating bulkhead. Your best bet is to get to the airport early and try your luck at the counter, maybe that can work.