Teaching her how important it is to eat healthy is just as important as not worrying about cleaning her plate. Breakfast is super important for kids.

The best way to teach her about cleaning her plate is to say "I've put a bunch of food on this big plate here. Put one or two things on your plate and when you are full you don't have to eat any more." When she is old enough to understand (I don't know if she is yet) you can talk to her about why you are concerned about it based on your own experience.

Of course the VERY best way to teach her to clean her plate is to make her do the dishes! Only a few more years! Why we all secretly had kids!