*As for the police, it was a stupid game gone wrong. Yes she’s under 18 but she’s in no way a child in the way people seem to be saying on here. She’s old enough to get married, to drive a car, to have sex. Whether we like it or not she’s old enough to do stupid things now and regret them later.
The police absolutely will not consider this an assault on a child. And even if the OP does go to the police, the girl will have to give a statement, and without that the police won’t do anything.*
Additionally incorrect. Whilst seventeen, she’s still the victim of a crime. She doesn’t have to ‘press charges’ or anything, the police will come, see a minor who is being treated in hospital with retrovirals and the like and have a duty of care to at least intervene. At an absolute minimum they will do a check on this girl’s house.
In cases of domestic violence, you don’t have to press charges for the police to intervene, they can contact CPS without your consent. Same in cases of assault/GBH. If OP’s daughter is in hospital with aggressive stab woundings and the girl has no visible injuries that clearly makes her an aggressor. It’s then up to the officers to establish whether it would be in the public interest (as is often the case) to proceed with intervention.
However as above, it is best to contact the professionals regardless so that they can take over how they see fit. Not to completely absolve them because it ‘doesn’t seem serious enough’ or ‘they wouldn’t do anything’.
We also don’t know whether Op’s daughter wouldn’t intervene. At that age I would have begged my mum not to intervene but in the presence of actual police officers I would have resolutely cooperated and told them the absolute truth. Which is all she needs to do, whether the police will intervene or not is not the point - this incident needs reporting.