Thanks for all the replies
Well I plugged it into to be charged last night, and it appears it wasn't really empty, because it was only plugged in for about 30 mins (while we transferred some books onto it) and after that it was showing battery almost full.
Just to clarify - in the first week of owning it, she hasn't added any books, read any books, used the wifi (although it's on) etc. We've been waiting for the cover to arrive. So it was turned on (after the initial charge), and then maybe 4 or 5 (pre loaded) books were opened, a couple of pages turned and that's it.
I've instructed her to power off at the end of the day after she's finished reading for the day, when she's next around I'll turn off the wifi too (not currently being used as we have 'hidden' networks, and it doesn't work on them?). We loaded a book onto it last night for her to read, so we'll see how long it last when she's actually using it.