Periwinkle, you may check that your dog food is nutritionally adequate. Foods with fillers and no good protein source cause dogs to eat more and be hungry more often.
Misty, I wouldn't try to feed her any more than she's eating (including the treats). She won't starve, and as long as she's on a good quality kibble (I've heard mixed things about Royal Canin. Believe it has corn, which is a filler), she's getting all the nutrition she needs. You don't want her to be overweight at all, especially while her joints and such are still growing. You might try swithing foods and see if there's another one she likes better. She'll eat very little of a high quality food, and it'll do well for her.
My 7 and 11 lb. babies eat 1/2 cup a day, so I don't think your little girl needs that much.