You guarantee nanny's pay for 50 hours and you add additional OT as needed.

Example contract language for a job paying $10/hr straight time, $15/hour overtime: "Nanny will work 50 hours Monday through Friday (outline hours for each day). Nanny will be paid a weekly rate of $550 ($10 per hour for the first 40 hours, $15 per hour for the last 10 hours). Any additional hours worked will be compensated at the overtime rate of $15 per hour. Family guarantees nanny's weekly pay will be $550, whether she works 50 hours or less each week."

I think you are better offer guaranteeing 50 hours at a lower rate than setting a higher rate and using the OT hours variable frequency. Nannies are more interested in the guaranteed weekly gross and making sure the schedule is reasonable. You really can offer the same guaranteed weekly gross for 50 hours as you would for 45 hours even though the actual hourly rate would be lower. A nanny who is looking for $700 a week for example is not going to turn down your offer because its 50 instead of 47 hours.

By setting the rate lower and guaranteeing 50 hours you will also make the nanny happy when she gets more extra time off when you don't need all 50 hours. You also know she is available since you are guaranteeing 50.

We want to hire a nanny for 50 hours but the 10 hours of overtime will be variable. Is it better to negotiate a weekly salary that may include the overtime or should we discuss an hourly rate and then pay her time and half over that for the extra hours each week?