Dieting for 8 Year-Olds

My 8 year-old daughter has put herself on a diet.  She is a big girl, tall and sturdy and strong.  She's the fastest runner on her softball team (and I have the times to prove it).  The pediatrician said her weight is fine and that people just come in different shapes.  But her friends are those tiny wraiths with toothpick arms and countable ribs.  So she put herself on a diet, which scares me to death.  She said she wants to eat healthier, and who could complain about that?  She's been chosing to forego her evening snack, and certainly nobody really needs one.   A few days ago, she said, "Mommy, look at my belly.  When I was chubby, it was out to here, and now I'm skinny.  I'm so happy!"  I gulped and said, "Honey, you know I don't care about that.  I just want you to be healthy and strong.  I always think you're beautiful."  But I know how I feel when I'm skinny (and I know how I feel right now when I'm not).    What am I supposed to do?

Last night was Halloween, and my husband and a few neighborhood dads took their daughters trick-or-treating.  One of the dads mentioned to my husband (in I assume a congratulatory tone, though I didn't ask because I didn't want to get even more pissed) that he could tell that our 8 year-old had lost weight.  Fuck him.