"Shh," Hermione soothed her daughter. "It isn't your fault. Rose, love, how could it even be your fault? Nothing about this is your fault." Not that she was even sure what 'this' was, exactly, which led her to her next point. "I can ... try to undo it if you wanted to."
It wasn't as though she'd never tweaked anyone's memories before. She'd done some very complex magic on her parents, ages ago, and she'd undone that. Perhaps it wouldn't take too much to restore Rose's memories. "How much time do you think you lost?" she pressed gently, stroking Rose's hair back, running her thumb across Rose's cheeks to brush away the tears.