You are not going to like to hear what I have to say on this, but your kid is way more important than this dog.
Yes it is dog-aggression and NOT people-aggression, BUT, if you kid is home and accidently releases the one dog at the wrong time, he can be as injured or more injured than your husband was, and I would not risk that.
I feel bad about this. I think it isn't resource-guarding because you haven't had her long enough to be that important. But I can't really be sure of that. I think this is same-sex aggression. Some bitches just do not get along. They are sneaky and violent, and you have already had serious injuries.
Spaying a bitch MIGHT reduce this if and ONLY if it was due to the elevated hormones during the heat. But bitches have memory. They are jealous, and once they decide they do not like another bitch, it is really NEVER safe to leave them alone together, or to let them lose unless you have two adults, not afraid to do what it takes to get them separated.
Why would you want to live life like that?
Spaying the bitch MIGHT make things worse as it can make bitches more reactive. And I don't think it helps with SSA at all. But that is just an opinion.
At this point, there is a 10-year-old child who is both in physical danger, if he tries to break up a bitch fight, and emotionally it can seriously do damage to how he feels about dogs in general.
Get rid of the new bitch. Send her back to where you got her from, or find a good rescue, or take her to a shelter -- last choice to be sure, but what is important, the most important is your kid's safety.
This dog isn't a bad dog. But she isn't right for your family.
Give it a few months, spend some time with each kid taking the dog you have to classes. Sign them up to train her in agility, or fly-ball, or herding. After a few months or even a year of working closely with the bitch you have, then go for a dog, or a dog-pup.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC