By the time mom could even react baby was dead and no longer moving so dog had also stopped. Fast powerful dogs! As a mom I would have taken the dog out before any law enforcement had a chance to respond. Need more info.
I'll admit to not knowing a lot about rotties, except my business does a lot of work with rottrescue.org (shameless plug for a great group of people). But I'd have to think there'd be some warning, possibly ignored. I've known many people who ignore aggression against kids. I was lunged at by a GSD when I was about 6. Luckily, the whole family reacted quickly. My aunt and uncle had the audacity to suggest it was my fault since I looked at the dog.
Of course, I denied looking at it since I had been told not to. But I did look at it. But I must have known even at a young age that such a warning was the height of foolishness.