So with an animal that’s showing unexpected behaviour it’s a good idea to brainstorm what could be going on, then slowly work through them, ruling things out as you go.
if she doesn’t want to be exercised, this indicates to me she might be in pain. Also this would tally with the sudden lashing out at you/your child. (Protecting her body for fear of more pain, or trying to tell you she has a problem)
do you have a good vet? If they can assess her thoroughly, if they can’t find anything sometimes they will agree to a ‘pain relief trial’. This is where you give pain relief for a week or two and see if the behaviour improves. If they don’t suggest it, you could bring it up.
is she six months old? If she is, reaching sexual maturity could cause her pain/hormonal issues )just like us!
Be aware that not all vets are great with pain stuff (sadly much like human doctors!!) so if you don’t click with the one you’re given, it’s worth asking for another vet or even trying a different practice.
Once pain is ruled out (definitely pursue that route first) I would search out a good dog behaviourist to come and observe her at home and chat through her circumstances.
I really hope you can figure it out, dear thing, sounds like you are great owners so there’s hope :) x