Students do this too. I second Gumtree and netmums, but definitely worth asking around anyone you know who might know any student nurses/teachers/vaguely child related people home from uni for the holiday.
I was thinking the very same thing recently (have to get a full time job soon and ds starts school in September). Great to know that such people exist and if Fibonacci isn't in the Derby / Notts area, I am and would potentially be looking for a school holidays nanny in 2010, so would be very interested in meeting you in those circumstances dibdab! (sorry for the hijack!)