The Charmed Children of Rookskill Castle by Janet Fox
My rating: 4 of 5 stars
Creepy (perhaps a bit too creepy for younger readers) and a little more about why the children were sent away (American readers won't know about the Blitz and child evacuations). Beyond that, what happens to the children - all of them - at Rookskill, and how they react is a great blend of realism (rebelling against an older sibling) and fantasy (Lady Eleanor's plans for them).
ARC provided by publisher.