We did that a couple of years ago when our daughter was about 4 years old. She was not waking up during the night and the nanny was staying for the weekend so we agreed a general price for the whole weekend.
In your case, i would say that £9 per hour when she is awake makes sense but that if she is asleep and the kids are too,you can reduce that down to £6 an hour.
Hope this helps
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