You will essentially be whistleblowing and there is a significant risk you will be sacked, or retaliated against in some way - e.g. falsely accused of meanness yourself.
If she's comfortable enough to openly behave like this - especially in front of new staff - then this is probably common for her. Other staff have therefore probably seen it and either aren't particularly bothered, or don't want to put their own heads above the parapet. Also, she probably does worse when nobody is looking.
It's your word against hers and sadly they will be inclined to believe (or protect) the established/senior member of staff over the newbie.
However you absolutely must do something about this horrible misconduct, just be prepared to protect yourself, have another job lined up, etc. I don't wish to scaremonger as the management might be perfectly kosher, but you need to protect yourself for the possibility they won't.
You need to make sure this is taken seriously because if they choose to not believe you or to give you the boot, the children will actually not be protected and your report will have come to nothing. She will be free to continue, or will just be more discreet about her cruelty in the future.
When you report to the manager, make clear that you are reporting this to the LADO and Ofsted, and that you will be handing in your notice if this is not addressed. That mitigates the power the hold by threatening to sack you, and by already reporting this to the LADO and Ofsted you will force their hands into acting.
Explain to the LADO that since this is an established staff member who behaved like this openly, and that you were the only witness, you are concerned the management won't do anything about it and fear retaliation.