Snickers25, the advice is that this woman needs to be removed from your family home. No matter the fall out.
She is not working, so she needs to go back to Oz ideally on the very next available flight.
Your daughter has been hurt on more than one occasion and she seems totally clueless as to why you are shocked by it.
You are not negligent. This woman CHOSE to harm your baby. That was HER decision.
Since then I kind of get where you are, you are in shock, perhaps too trusting and too worried by what reaction your protecting your children will cause. This is a common reaction in child abuse cases. I say common, by no means does this make it acceptable, but you seem paralysed to act, for fear of rocking too many boats.
In short, BUGGER the boats!
You need to focus on what has happened in it's simplest terms. You need to repeat it over and over and not get involved in any form of discussion/reasoning.
"My daughter was deliberately hurt by this woman, at least once, very probably twice, if not MORE, and it is not ever acceptable."
Realistically she needs to be on a flight in the next few days. Tbh, until then she ought to be in a hotel/B&B, but I understand that may be nigh on impossible to achieve.
You need to communicate to your DP and to her that she will not be allowed to harm your DC, and that you are outraged. If she doesn't agree to go as soon as possible that you WILL be considering calling the POLICE and having her arrested for child abuse.