If care is only needed term time, then I think a term time nanny may be your option.
The issues I see are:
The early start time, if you need to leave home 6:15am
The late finish time I do not feel is such an issue as personally I do not mind working lunch till late, though for some nannies they may not want to work that late.
You need care for all your children, who vary in age (I presume) and may therefore need taking to / collecting from school, taking/collecting from after school activities such as sport related or youth group.
Some of your children may feel they are too old to be with a childminder.
Positives of a nanny:
Care at your home, so no need for the children to leave home early morning, or come home in the dark in winter.
Not just childcare, nannies will often do additional tasks such as childrens laundry, food prep, helping older children with homework.
If a child is mildly ill, they can stay at home (may need to be transported to other childrens activities).
More focused around the needs of your family, so if one of your children needs to go to afterschool swimming lessons, its not a big problem.
Negative of a nanny:
Cost, are you sure it is similar to other childcare options you are considering?
One provider, if theys are sick, no one to cover.
Time keeping - you need someone very good at being on time if they live-out, especially to be at work by 6:15am.
Au-pair and childminder combo may work but it depends on the ages of your children? If all school aged it may work but if one or more are a lot younger, then it may not work so well.