When my DD was being bullied by a boy in her class (she was throttled on a near daily basis, she was punched, she was kicked, she had things stolen out of her hands by him...) and her school refused to do anything about it, saying "oh, he has SEN...", I went to the police. Who dealt with it brilliantly and sympathetically, to both sides. They were 9/10 at the time.
When my DD was homophobically bullied on-line by a boy in her year, and her school refused point blank to do anything about it... I went to the police who raged at the school on my behalf and helped me to take out the equivalent of a restraining order against the boy to protect my DD when it escalated to threats of violence against her during school hours and... the school did nothing. They were 13/14 at the time.
When my DS was bullied and had death threats made against him last year, aged 10, and the school did nothing, I went to the police (different county force). Who dealt with it sympathetically and - from what I was told - got the very unhappy child who was bullying/threatening him the help/support that he actually needed (and he was very unhappy at home, by all accounts).
Your child has been physically assaulted.
If you physically assaulted her, the police would be involved.
If a stranger physically assaulted her, the police would be involved.
If another adult physically assaulted her, the police would be involved.
If a teacher physically assaulted her, the police would be involved.
Just because this is another child doing it, doesn't mean that they don't know what they're doing. And, frankly, if the police have a few words with mum/dad/them... it might stop them from ending up in even more trouble further down the line.
My advice, OP, is ask the police for their advice. Schools are loathe to do anything that will interfere with their Ofstead results, in my experience/opinion, which means brushing bullying under the carpet and turning a blind eye to it, whilst claiming that they have a zero-tolerance police towards it. Don't, for your own sake, confront the parents. As a pp has already said, they won't believe you and it will only make things worse. For your DC.