I think you need to meet the dog, and see how your MIL deals with it before you make any decisions.
Does she know the dog's history? Did she get it from the breeder and meet at least one of the parents? I would be loathe to have a dog whose history I wasn't aware interact closely with a child until it had been properly trained, and even then I would be wary.
I've got two rottweilers. They are wonderful with my 13 month old, BUT they are only allowed anywhere near her because we dealt directly with the breeders, met the parents, and have had them both since they were puppies. They have been trained hard since the moment we brought them home, and as a result are obedient, calm, and affectionate. I realise that not everyone on MN is a fan of dogs (slight understatement there!) but I think a relationship with the right dog can be really beneficial.
My daughter adores our dogs, and spends hours playing with them, Girldog especially. Girldog is happy to lie on the floor while little Ff crawls all over her, pulling her ears and stroking her. Rottweilers get bad press, but, I see a lot of dogs and wouldn't hesitate in choosing another rottie, if, God forbid, anything happened to one of ours.
Don't immediately jump to the conclusion that because the dog is a rottweiler it is going to be savage and dangerous. ALL dogs have the potential to be dangerous, and you need to judge individual dogs on their individual personlities.
I think you should talk to your MIL before it becomes a problem. People can get very emotional and defensive about their pets, so even if she is understanding now, a situation might develop where she thinks you are being unfair.
Get to know the dog, and, please feel free to ask if you have any questions about how Ihave dealt with my rotties.