Hello R..
You've gotten a great deal of good advise here, and I don't think I can improve upon it.
I agree with those who have said you're doing fine in disciplining your pet, and protecting your child. Aside from it being a bit chilly this time of year, no harm can come from taking your pet for extra walks or from rewarding the behavior you prefer from your pet with treats or extra attention. I would consider it wise as well, to restrict your pet's access to your baby during time on the floor until you are certain that your dog has adjusted to your new addition, and to continue to monitor your pet's behavior as your baby goes through his milestones.
This is not to say that there is no risk. There are no guarantees. However, logically, we know there are millions of children growing up with millions of dogs and surviving to adulthood. I see no reason in your situation to assume that's going to be any different or to rattle off the statistically rare horror stories of children being harmed by previously presumably non-aggressive dogs. Given the amount of interaction you have compared to what your dog has, your child has more likelihood of coming to harm at your hands than he does from your dog.
I think it's alarmist and extremist to state that you must get rid of the dog immediately for one growl. Everybody in your household is going to have growing pains adjusting to your new baby, including your dog. It does not sound to me like you have a high risk situation occurring or that you are dealing with an aggressive breed. Reasonability would indicate keeping an eye on your dog around all people including your baby for the time being. If the behavior increases, get some help and take more precautions until you can. Though I'm not an advocate of the Dog Whisperer, some of what he has to say may give you ideas about how you want to handle your situation. You may also like The Dog Listener. She's from your side of the pond, and I prefer her methods.
My son is 4. I have had 4 Border Collies since before he was born. Once he was born, I had varying degrees of the same behaviors you're seeing with all of them, from increased guarding the house, guarding me and the baby, guarding only me, guarding only the baby, guarding the baby from each other and other people, and guarding the baby from me, etc.. To this day, they are alert and aware of where he is every time he's around. Though they have growled at him, especially when he was on the floor and when he was first learning how to pet them, they have never bitten him or anyone else. He now leads them around, helps feed, wash, brush and walk them, and has them taking turns sleeping with him in his room. They are his "friends", and he learns a lot from having them around.
Hang in there. I think you're all going to be fine.