Well, I'd ask about food - will she be giving him breakfast? If so, what? Are there things you do or don't want him to have?
What about after school - again, food is a good place to start. Some CMs give them a drink and snack when get in, but others want to serve a meal before you pick them up.
Make sure you understand the contract - what will happen in school holiday? Do you pay a retainer / will you be wanting her to look after him in school holidays ?
How many other children will be there.
Does she have pets? If so, what? (presumably more important if your dc has allergies or a real fear of dogs or something)
As he gets older, what happens if he wants to stay for a club or practice after school, will she be able to fetch him later?
What will your time keeping be like? - Do you have a long commute? What happens if you are unavoidably delayed ? (Or what happens if you know now your hours will be irregular)
What happens on INSET days ? Does she tend to have children for those days or not?
How would she want you to pay her?