Start with the duties.
Childcare at all? If it will feature at any point in her job you'll need to point it out at the start. It would be easy to take someone on thinking it's just for the house, then try to start using her for a little bit of child care here and there, "just while they're watching tv and you pop to the shops" but that sort of thing grows like topsy and can become An Issue.
Secondly, cleaning, cooking, shopping, inextricably linked with timing and hours. Where do you most need to keep control and where do you most need to relieve yourself of effort?
Shopping will involve handling money. She will need to know this.
Divide those up. Where to clean. Everywhere? Is she to use her initiative: for example, emptying the fridge, defrosting the freezer, cleaning the sofa covers, or will you set up a routine?
What to cook? Is she to be there early, for breakfast? Is she to cook an evening meal before she leaves for the day? Is she to use her initiative on what to prepare?
Flexibility. Discuss at the earliest point. If you need it, do not take on someone who says they are not very flexible and hope to work around it. If you want flexibility, decide on the number of hours at the start of employment, not the number of days.
Make everything as clear as possible before you start. Meet her at least twice and if possible, go to her house. That will give you a good idea about how good a housekeeper she is.
Word of mouth is good but not infallible. Find out what they did in their previous job, write it down, think about it later and then tell her what will change about working with you.
Be clear at all times. This is a major level of trust and you need clarity and honesty on both sides very early on.