Oh, but that's a great age, there's so much out there to buy for us them!
Theresa Thomlinson probably more like 9 (or older, but I was dead sheltered!) than 7. What is lovely is the Children of Green Knowe by Lucky M Boston. It does have a male main character, but two wonderful strong female characers too, and it is a lovely book. If they like that, Chimneys of Green Knowe has a main female character.
(Maybe you know these, sorry! I just get giddy about books as I love them!).
Alan Garner's books are great too, IMO, but lots are for older ages.
Do you know Wynne Jones's Magicians of Caprona? I love that so much. I think the thing I like best is that it re-tells a Romeo and Juliet story, but does it so as to present boys being friends with girls as normal. I think that's great. Cos I was thinking, of the books we're talking about, you don't get many boy-girl friend pairs, do you? Siblings, yes, but not often friends.
Same in grown up books, I suppose.