elliott - I have that sort of arrangement, but different from what you want in that it isn't just 2 days a week and occasional holiday days: mine is 5 days/wk and full-time in holidays.
Point in telling you this is that the big issue with after-school jobs is that they are only part-time, so can't pay a living wage (unless you make it bogglingly well-paid for the hours) - and as absolute natural consequence of that, your catchment of applicants is limited: to people who (a) have another PT job/other kind of commitment & want to fill in the gaps (which naturally affects their flexibility); or (b) don't actually need to rely on your job but are doing it for extra pocket money (which might affect their reliability).
Which means that after-school people, even closer to FT like mine, are hard to find ... and your arrangement might be even harder. (PS - re pay: I do pay a nearish FT rate to my nanny, smoothing out a proper FT rate in the holidays to beef up her PT rate in term-time.)
But - that's not to say it's impossible - you can get lucky - like LM's friend - the "local granny nanny" is often the solution. Or student - are you near any colleges?
On the other side of the coin, if you were looking for PT morning help you'd be inundated with applicants, as those hours are when all your local au pairs (which, of course, all do the after-school stint in their host families) are free.
So ... have you considered an au pair?