I'm confused about the hours:
She would have to be up at 6.15 for school runs, not have the children again until 4pm for 10yr old and 6.30pm for 12yr old.
So 6.15am to what, 9.15am? Then 4pm (is that collect time, so maybe 3.45pm leave the house) to 7pm when your DH gets back.
So 6-6.5 hours per day perhaps.
I work four 12 hr days then get three days off, then work four 12 hr nights then get seven days off. Most of those days off I would give the Au Pair off.
So au-pair would only work the days you work but those days will vary. How will you explain this to the au-pair - some kind of calendar perhaps?
If I get this right it would be like:
work 4 days, then 3 days off, then four days (or is it 5 as you would be sleeping after night shift), then 7 days off, then 4 days, then 3 days off, then four days (or 5), then 7 days off. And on and on and on.
Up at 6.15am... not sure how many au-pairs would like that. I have no experience of au-pairs but as a nanny I get up earlier than that some days and it is a bit of a struggle.
Does DH work shifts like you? Maybe it would be better to fix the hours that the au-pair is on duty, such 6.15am-9.15 and 4-7pm Mon-Fri... is that possible?
The 25 hour limit applies to au-pairs from Romania and Bulgaria. So probably avoid those countries, instead look elsewhere in the EEA. That way you can agree the hours with the au-pair and not worry about working hours restrictions due to visa/permit.
I am unclear as to why I would still need to pay holiday when in effect they are being paid for nearly half a month off.
You say you will pay £70 every week, so if you are doing that 52 weeks a year then you are paying holiday and you would be giving them their statutory holiday entitlement, exceeding it. However would the au-pair be able to choose any of their holiday? They may want to go home to visit family and thus take a week off or longer.