Here is the post (well, two posts combined actually!).
Was going to say Goodnight Mr Tom but someone beat me to it!
However, I will add 'Back Home' by Michelle Magorian (same author as for Mr Tom) - I preferred it. It's about a 12 year old returning to the UK after the war having been evacuated and trying to find her feet in a country which is trying to sort itself after the war, as well as sort out a relationship with her mother. Fantastic book. the author wrote quite a few others which she might also like if she enjoys this.
Yes to The Dark is rising sequence (Susan Cooper).
A bit left-field but Just William? Funny but the writing is quite sophisticated.
L.M. Montgomery - Anne of Green Gables but also Emily of New Moon series.
There's a wonderful follow-up to 101 Dalmations which most children have never heard of: The Starlight Barking (Dodie Smith) - a day comes when all dogs wake up and humans don't - there's a reason for it and it ends with the dogs having to make a collective moral choice.
If she liked school stuff, the Trebizon series? more advanced than Malorey Towers!
Noel Streatfeild - I loved them at that age - easy to read but I loved the stories.
More adult - My Family and Other Animals - it was when I read it and it started the love affair I had with Gerald Durrell's work.
Tom's Midnight Garden (I still quite like this one!)
The Indian in the Cupboard series.
And there was one called Moondial by Helen Cresswell that I loved at that age. I even pretended myself into a game of it, and then (because it's a bit spooky) got stranded at the bottom of the garden one night having imagined certain things into 'reality' - I couldn't bring myself to run past them back into the house!
None of mine are modern - they are all books I loved at around that age. And some of them were old-fashioned then! All damn good stories though.