is there anything they won't do - i had 1 who didn't do windows? Do they have any health issues that might affect their job? do you know where they live and are planning to get to work (eg are they reliant on a friend giving them a lift). are you going to ask and check up on references? how long have they been doing it for? Maybe ask them about how they would go about cleaning something unusual? try to get a feel for what they do with the rest of their week - ideally you want someone who is a committed cleaner who's used to dealing with lots of dirt (you have 3 kids) who needs the money. Not someone who's doing this as a hobby or who cleans for old people. have you agreed terms and conditions such as pay rates, increases, holidays. do they have young children and want summer off? are they insured or are you covering this
Not suggesting you want to ask them all as questions but these are things that could affect their service.