I had a dog just like this. The growling and nipping already is a real red flag that this dog could do some damage to a younger child. When you have two kids, it is MUCH harder to keep an eye on a dog that you can not trust to be alone with your kids.
Training is something you can try, and each dog is different, but after 2 years of us trying to train both the dog and my child (who was BEYOND gentle), having an animal nip at him which will lead to biting if the child does not immediately leave the area of the dog---well, let's just say I did not want to take that risk. And as a Mom, you do blink..and in that blink a lot can happen.
Keeping the age of the dog in mind, it is unlikely training will help...The above poster is right that with age, medical problems arise that can cause her to become more agitated, and I can tell you from experience--the majority of old dogs have an attitude or are moody. (Vet Tech before kids)
Well, you have to decide just how much of a risk you are willing to take. Dogs are not human--they still retain wild animal instincts when they feel threatened. With you being pregnant, and adding yet another "threat" to what your dog already sees (your son), things are more likely to get worse, and not better.
Keep in mind that Chow, as a breed, is unpredictable...Lab is loyal, Greyhound is fragile and tends to be cautious and anxious...and you have already stated your dog is already nervous....adding another child will only make that worse in most dogs....
We actually had to make the hard decision and we decided it was safer for the dog to find a new home. She now lives at the beach with a husky as a sister.
Bottom line is you have to listen to your head and try to hush your heart for a moment...go with your gut feeling...because it usually is right...I didn't listen and my son got bit...