Honestly, as a nanny, I wouldn't expect a pay raise in this type of situation. You're paying for 40 hrs + OT while she works 30. I'm assuming you give guaranteed pay (pay whether or not you need her) and PTO?
Nothing wrong with having a sit down. If it becomes an issue, you can find someone else who would be happy to work 30 hours a week with guaranteed pay + OT, with a lower starting hourly rate than what she has currently I bet. Good luck!