Knight's Castle (Paperback) by Edward Eager

If the old toy soldier hadn't come to life, Roger would never have discovered the magic.
     And that would never have happened if he and his sister, Ann, hadn't been sent to stay with their bossy cousins for the summer. And that wouldn't have happened at all if their father hadn't gotten sick and gone into the hospital.
     But all of that did happen, and now Roger, his sister, and their cousins find themselves in a bygone world of chivalry and knighthood, of Robin Hood and Ivanhoe. In this knightly realm they can make a difference—and perhaps even save the person they most need to save—if only they are smart and brave enough, if only they are true to their hearts.