@anadascal Firt things first- I'm so sorry to hear about this situation. You are in an incredibly difficult position. It sounds to me like you are approaching it with deep care about safety, and lots of love. No one could ask for more. Thank you
I want to echo the sentiments of other commenters and affirm your own intuitions- this is a very very big deal. Jennie is feeling uncomfortable/unsafe, and is escalating her aggressive behaviour to meet her needs. The next bite could happen earlier in her stress build and/or could be much more serious.
How do you get Sarah to see that this is a big deal? Sarah's attitude sounds very troubling, and she does need to change her thinking. But it may help to start with where she is right. I think Sarah is absolutely correct that Jennie's aggressive reaction was understandable. I'm guessing Sarah isn't great at noticing/understanding Jennie's warning signals, but she clearly has some understanding that her dog is very stressed by her toddler, and that stress is what motivated the bite. This is all true! And valuable information! Sarah's empathy for her dog's experience and perspective is not a bad thing.
What is a bad thing is to minimize the seriousness of the dog's stress, and the likely consequences if that stress is not managed/ changed. Jennie may have already begun to learn that she is not safe in her own home, and that the only way to get her needs met/ have her boundaries respected is to bite. Jennie may be very likely to bite again the next time Zac touches her in a way she does not like.
What it would take for Jennie to remain in that home safely:
Caveat: this is just a layman's high level, back of the envelope list. A trainer who is an expert in aggression should do a full assessment. Additionally, it may not be possible at all for Jennie to live in the home safely, even if the below is in place.
What concerns me most is that Sarah said she'll be "getting Jennie retrained". It sounds like there is a lot of retraining required in this situation, but most of it should be for Sarah. If there is any hope of that dog continuing to live in that home safely, Sarah needs to become an expert in her dog's boundaries, needs, emotions, communicaiton signals. She needs to learn a lot about aggression. She needs to become an expert in management and safety protocols. Sarah needs to advocate for, manage, soothe, support, coach, and train Jennie. Every single day. If she doesn't, Jennie will not have the help she needs to feel safe in her home, or learn new behaviours. There is no board and train or puppy class that can fix this. It has to happen every day, in the home, with the family.
As for Jennie's training: Jenne will need to be slowly socialized to Zac. That means short infrequent interactions that cause Jennie zero stress, and NO OTHER INTERACTIONS AT ALL between Jennie and Zac. Ever. It will be painstaking, slowwwww, and must be very carefully managed. Zac must never be allowed to touch Jennie in a way she doesn't like. Jennie also needs to be trained to choose other behaviours when she feels stressed (could be flight training, rebuilding warning signals, choosing a safe space, etc). Importantly- the vast majority of that training needs to happen in the home. Sarah needs to be the trainer. Sarah and Jennie need to develop an extremely strong bond of mutual trust and collaboration. Jennie needs to trust that Sarah will help her feel safe and protect her boundaries. As I said above- the owner must be the one to do this work. Dogs like Jennie need constant support from a skilled guardian.
And, while all of that training is going on, management must be absolutely flawless. Dog and baby must be completely and securely separated at all times, except during structured training sessions. Muzzle use is probably a good plan (that involves slow and careful muzzle conditioning to be added to the training list). Jennie should get lots of enrichment every day, and be kept away from any other triggers she may have. Jennie's life should be as stress free as possible, to set her up for successful behaviour modification.
The cost of all of the above can be immense. As I said above, Sarah needs to work closely with an expert in aggression. She should also speak to a vet about behavioural medication, and possibly consult a vet behaviourist. She will need to modify the infrastructure in her home to make it secure with sufficiently sturdy locking gates/ door locks. She'll need to buy a muzzle (many of us end up having to buy a few before we find one that works). She may need to buy equipment to help Jennie with her emotional life- could include enrichment toys, white noise machines, etc. Sarah may need to hire help for walking Jennie, given how much time will have to spent on everything else required to improve the situation (important that any dog walker is R+ only, is experienced with aggressive dogs, and takes Jennie on decompressing outings). But I believe the biggest cost will be in time spent, opportunities lost, and emotional energy. Sarah will have to be vigilant 100% of the time. Rehabilitating Jennie while keeping her baby safe will be unrelenting, exhausting, high stakes, emotional work. The cost of a mistake is her kid's life. She will have to show up emotionally and pragmatically for her dog and her kid in a very intense way, all of the time.
Ideally, it wouldn't be that extreme forever. Ideally, behaviour modification works and you develop some good management strategies that work for your life. But that is the ideal, not a guarantee. And it would take months or years to get there.
Takeaway:
So do you need to try to convince Sarah to rehome Jennie? I don't know. But someone needs to help her understand what it would take for her to keep Jennie. Someone needs to help her think realistically about this situation, and wrap her mind around how incredibly serious it is. I know a couple of incredible people who safely foster or own aggressive dogs and have children. It can be done. But it is not for the average bear- it is for the extremely committed, educated (or willing and able to quickly get educated), passionate person who wants to dedicate a few years of their life to rehabilitating a dog. That is THE hobby, THE weekend activity, THE topic of your books/ podcasts. If Sarah wants to and can be this person, then I commend her.
Try to have some compassion for Sarah as you speak with her about this. It sounds like she loves Jennie, and knows that Jennie isn't being set up for success. There is a lot of grief and pain for a person in Sarah's position. Blame and shame are not likely to help her get her thinking where it needs to be. Empathize where you can. Let her have room for her feelings.
But please for the love of that little boy, don't drop this. This is a very dangerous situation. No one has to label Jennie a bad dog, or Zac a bad kid, or Sarah a bad dog owner. But this is a BAD situation. It must be resolved, now.