Will the nanny need to be on call whilst the children are in school? If they are ill and need picking up? If there is a teacher training day/broken pipes or can you cover those times? If you need her to be on call, then you need to pay her for those hours.
I looked after 7 year old twins for a year and with holidays, sickness, training days and snow days, out of 260 week days, they were off for 148 days. Granted, they were at private school so a few less days off for state school.
Their school day was 9am to 3:30. School was 15 minutes away (30 in rush hour) I dropped them then went either to the supermarket or run errands (drop/collect dry cleaning, speciality shops (health food, electrical for light bulbs/fuses/tools, post office to collect/return parcels etc) or home to make/strip beds (I stripped my employers bed but the cleaner remade it, huge bed and duvet, took me ages to make it. I also told her that if I ever found anything grim in it, I would not finish the job nor do it again! Heard many, many horror stories of disgusting things found in beds and laundry baskets so I also got her to put their washing in the machine and turn it on and I would dry and fold it once I knew it was clean!) tidy the boys room (they were pretty tidy so not a great deal of tidying), dust and Hoover it on a Monday. Empty the dishwasher and load breakfast things. Prepare after school activity equipment (tennis, swimming, rugby bags) or an activity for us to do together usually the beach but crafts or baking on rainy days or for birthdays, Christmas etc. prepare after school snacks and drinks for them. Prepare tea for them and sometimes employers, depending on what I was making, batch cooked for the freezer. Do their washing, ironing and putting away (and they had loads of it! Clean uniform every day, tennis kits twice a week, swimming towels and muddy rugby kits and after school clothes (they were pretty messy boys!) mending/darning/sowing in names tags. Recycling (didn't have lorries then) sorted and stored too small clothes, bought new clothes as needed (mainly school/sports clothes, pants and socks, after school clothes were done by MB with the boys) apart from bosses laundry and bedding, I did all of these jobs before the boys started school!
I did meet friends for coffee or lunch a few times a week or go to the gym or swimming (lifeguard had my mobile incase school called and leisure centre/beach was 2 minutes from school)
Some days I had barely 10 minutes to myself, other days they were pretty chilled out, very much like my role now with an 11 month old!!