Hi,
I have two dogs a Rottweiler and a miniature labradoodle and a now 8 month old baby.
My Rottweiler who is a genuinely lovely dog became defensive over the baby. We first noticed with my sister in law who he hardly sees. She came in and approached the baby who was asleep and my dog stood in between them and growled. This happened a few times with other people who do not often come to the house. Unfortunately this coincided with a terrible vets experience for my poor dog who was then a little nervous of new people.
I am a very experienced dog owner and both of my dogs are very well behaved/ well socialised and have good manners.
I took some advice from a trainer who we had completed our obedience training with who gave really good advice.
My dog is trying to protect the most vulnerable member of our pack, as I result I made it clear to him that he didn’t need to as I was the one responsible for protection.
If your dog growls you need to correct her straight away every single time. Keep an eye on her body language and nip in it the bud before it starts. If she continues you need to remove her from the situation (put her in another room) so that her actions do not get her what she wants. (The postman to leave).
With my dog I went back to basic commands walking to heel/ training sessions every day. I invited people round placed him in a down stay when he did not react he was awarded with a high value treat from the person who visited. I also advised people to ignore him when they first visited and to stroke him/ give him a treat when he approached them he has now built up his confidence again massively and we are not having any further problems.
My dog has never shown any unwanted behaviour towards the baby (obviously never left unattended) and is incredibly gentle.
Collies like Rottweilers are intelligent sensitive dogs you need to work with her. Dogs can become unsettled with new changes it’s not really a question of if you can trust her, she needs your help in understanding how she should act with this huge change.
I will say though my dog did not escalate to the level your dog has seemed to. He only ever had a low growl you may need some professional help in order to sort this.