Hi Athene, some thoughts.
I assume you are getting an aupair (hence the age concern), rather than a young nanny.
You have to ensure her level of English is high. Otherwise it will be difficult for you to give instructions to her whilst flying out the door or call her to give instructions over the phone.
That usually means an aupair from Western Europe, North America or the Antipodes. But such an aupair is likely to want to have a social life and could possibly baulk at the long hours. Having said that, I know of (German) aupairs who work for a particular family who use them for nannying duties whilst both parents are out at work all day (doctors). So they are out there, you will have to pay more, give perks and ensure their weekends and Friday nights are sacred.
At the risk of huge generalisation, I think Eastern Europeans could be keener on longer hours because that comes with more pay. And possibly less focused on having a good time. But they are usually here to improve their conversational English - so English is generally of lower standard.
Your best bet is to get an Eastern European aupair who has previous aupair experience in the UK or an English-speaking country. That is good too, for references.
I have a mature and sensible 19/20 year German aupair with good English. She is fab and will register my instructions that I throw at her on my way out the door. I have never used her for this but I think it would be a push to expect her to arrange holiday care and playdates beyond her meeting up with a few aupair friends in the park on an ad hoc basis.
She does what I tell her to do to a T but rarely beyond.
It sounds too much like an organisational nanny role if you expect her to open school bags, update you on school activities, ensure dcs are in the right clothing/have the right books for the right day, call up holiday camps and book them, call other parents to arrange long-dated playdates.
I am sure the right girl will do it but it is bit of alien territory for an aupair's skill sets. It could work if on the day before and the day itself, you remind her. But to rely on her to keep the calendar and buy that fancy dress in time for the play or make the poster for the school fare without prompting is a bit risky.
But then again, you might not be intending for her to take on those level of duties.
If you want a 'nanny-like' aupair, it becomes a bit strange if she is also asked to do general non-child-related cleaning and housework, which an aupair is generally expected to do. Different skill set.
Also consider the fact that aupairs tend to be a bit hit-and-miss in terms of finding the right girl. Even if you found the right one they don't tend to stay long term, hence revolving door. I would be quite wary about relying too much on an aupair to be anything other than an extra pair of hands around the house.