Great if you can get references.
I wouldn't let them keep a key until after a few months.
If you have them more than once a week, pay them on the last day.
Be clear about what type of cleanning you want.
Do you want them to sort out house stuff,toys,clutter etc. (great help for busy mums)
washing,iorning putting clothes away?
try and communicate your standards eg. base of toilet,sink,should they move furniture,remove cushions from sofas and clean? etc.
Go for a 4 week trial and both have a chat after that to see what you would like them to do more/better.Discuss hours.See how it is working for them.
be clear about how many hours and their break times.
hormonalmum you can use all these points from the other angle when you meet your future clients. I would be impressed if the cleaner asked if I wanted some of the above things done.