Just checked this with my housemate who is a Social Worker at Children's Services because I knew this was a safeguarding. He said to get your daughter medically checked asap just to make sure she's ok. Then contact your local council's Children's Social Services Dept to report your childminder because if you just remove your daughter from her care, she could do the same to another child. They might not be so lucky. Social Services will probably sign post you to Ofsted who register childminders and/or any other organisation who should be involved. NB, You won't be in any trouble, just your childminder.*
You are absolutely not being unreasonable, not just because the childminder deliberately let your daughter bang her head when she could have saved her. That is never acceptable. But also because her logic is seriously flawed. I'm playing devil's advocate here, but if the theory claimed that injury is required to learn some kind of lesson, what would the 'lesson' be here? That standing up is bad?
I saw an episode recently of Monkey World and they use this 'hurt themselves for their own good' lesson. The perimeter fence gives a mild electrical shock, which stops to stop the primates from escaping. It's not a behaviour they want to encourage because it's potentially dangerous for them and others for them to be wandering around Dorset on their own. So while they don't like doing it, it's in the best interests of the animals they love because the fence is the lesser of the two evils. Sofa surfing is a normal developmental stage in learning to walk, which is something that should be encouraged, not discouraged. The only evil here is the registered childminder who stood by and let a baby sustain a head injury. It may only be minor, but that's luck and certainly not down to your childminder displaying anything like good judgement.
The last thing you want is your daughter learning that standing on her own two feet results in pain. If she was doing something she shouldn't and the childminder let her fall so she didn't do it again, that would be more logical. Still totally wrong of course, but at least it would make sense. But if anything she should be encouraging and rewarding your daughter for pulling herself up on the sofa, not punishing her by deliberately refusing to prevent a head injury.
I personally would pull your daughter out of the childminder's care with immediate effect. If she says you're in breach of contract, point out that your contract means she is in loco parentis for your daughter and she failed to protect her from danger as her parent(s) would. She is the one in breach of contract, not to mention moral and ethically unsound and seriously lacking in empathy. I hope she gets struck off because she's a danger to children.