The breeder doesn't sound very good. You don't want to rehome your other dog!!
When you say the dog seemed settled before your DD stroked her, is it possible that she was settled as in, 'settled down and not wanting to be disturbed'? Do you know any of the materials on teaching children how to behave around dogs? There's some useful stuff by Sophia Yin here that you could chat about with your DD. You will have to supervise very carefully though. Also, if your DD is going to pet her, teach her to pet under the chin or on the side of the dog, not on top of the head or leaning over, and especially not to hug the dog. If you're not sure about it, then teach DD and the new dog to ignore each other for the time being, until things are settled down.
It sounds like you will have to take things very slowly until you can get a behaviourist in to help.
I know it can be scary when a dog growls at you, but never tell her off for growling. It's her way of telling you that she is unhappy with something, and that's useful information. What can happen with a dog that is punished for growling is that they learn not to growl - but still feel uncomfortable in those situations and so they just go straight to bite, without warning (or with only a subtle warning that most people would miss).
Is she eating properly? Is she motivated by treats at all? If so, then you can start using treats to reward her when she behaves well. One way of doing this is to count out a bowl of treats in the morning, put it on the table, and dole them out during the day whenever she is doing anything that is good or calm. It might just be when she is sitting calmly and looks relaxed, or it might be when she responds to a command from you. You can use some of her regular kibble instead of treats if she likes it, but most new dogs will need something more interesting, in which case you'll have to reduce the amount you are feeding her at mealtimes so that she doesn't get overweight.
Another way of doing it is to pick three behaviours that you like and want to increase, e.g. staying away from the table when your DD is eating, lying down calmly in the room and ignoring your DD, etc - whatever works for you. Then whenever she does one of those things, give her a treat. In so far as you can, ignore any bad behaviours as if they aren't happening. The three behaviours that you have chosen to reward will become more ingrained and happen more often; and once they are all fixed you can start to pick another behaviour to reward instead.
Best of luck. It does sound like you are in at the deep end right now; if you can get a behaviourist it will really help.