It definitely sounds like a bit of jealousy. I'm sorry your child got bit
I would start by making ALL positive associations with the baby. When the baby is out, treats/food comes out. When Triss is being calm and good around the baby, again praise and treats. Or maybe play toys with her while holding the baby, etc. Basically whenever the baby comes out, ALL things must be good!
You say she doesn't get free roam until the baby is asleep, and she's in the expen alot of the day. Maybe that's where she is holding her resentment. In her mind, baby = expen. Baby goes to bed = free time. So she may be learning that NO baby is a positive thing, whereas it should be the other way around. Also, her pen is becoming her territory, so the growling is a warning to back off. A growl can and often will be followed by a bite.
Is there a reason the dog has to be kept in the expen? Is she destructive, or something? Jackson would never stand for staying in the pen while I am home, lol. I've always let him be a part of everything so that he learns. He's grown up with a young girl (little girl is 2 1/2, Jackson is almost 2) and often sees my aunts baby who is 8 months old now. I never locked him away (of course except during potty training) for fear of what happened with Triss.
I wanted him to feel a part of the family and learn to be around babies and not feel particularly jealous. The little girl he lives with can easily take his bone away, put her hand in his bowl, be in his face and he doesn't care (not that we encourage this... we stop it when we see it of course).
Maybe when you take your walks, have the baby in the stroller with you if possible. Often times the dog learns to feel like a pack, both with human and other dogs, when you walk together. Basically, I would just start by making all things positive when the baby comes out.
I know it's hard enough with a young baby, and also having to deal with a dog, it's gotta be tough. But I think you can get through it with little steps. I wish you luck.
