Sounds like you are doing great with how you are encouraging them! It is so hard to see what others have that you want and accepting that you can't have it. I remember the days well.
Here is an article from our sister site SparkTeens.com that might help each girl identify their own body type which might help them move forward in their acceptance of it. A Teen's Guide to Weight and Body Confidence: What You Need to Know about Weight, Body Shape and More