FionaBeth, pleased to read your latest post here, it sounds as if dog and baby might be a great partnership.
Marko, you wrote ...
In general dogs rarely go from calm to aggressive in an instant.
True but I've had it happen and it wouldn't surprise me at all if the Lab thought it should protect the baby. I had a cattle dog that grew up knowing a friend and his daughter. One day we left her chained up alone(this was before I had my dog yards) because we had to take another ACD to the vet. So, the friends called in and the daughter went to say hi to Snoopy, who apparently acted overjoyed to see her for a bit, then must have decided no, I'm in charge and supposed to protect the place and according to the girl's father 'she went beserk'. As the daughter was backing away, she was snapping at her. My answer to that was that she was actually being polite about it, that she was fast enough to have bitten her if she wanted to. I would suggest that supervision would always be needed, FionaBeth, just because your dog is protective, not because it would ever hurt your child.