Just do cardio. If you're already slender, you will lose extra body fat and mostly trim up, but it will also help diminish the extra baby fat on your face.

You can't just do a facial exercise to lose weight from your face, otherwise chewing gum would work wonders.

It has to be a whole body thing where you get your heart rate up to the proper calorie burning zone.

Exercise is good for you even if you aren't trying to lose weight, it keeps your heart healthy among many other benefits. :)