The root cause of most eating disorders is a low sense of self esteem. Be very sensitive to your overweight child. 
 

Once upon a time there was a little girl who was innocent, playful and had the widest, most innocent eyes you had ever seen. Sometimes you'd see her giggling or laughing till the tears ran down her face, and other times you'd see her cry because her big brother bullied her. But most of the times you'd see her with a smile on her face, running in the sunshine.
 

One day something bad happened to the little girl. An uncle sexually harassed her, her mother insulted her cruelly and called her fat, or she was beaten up with a stick. This happened again, and again, till it developed into a pattern. The little girl slowly developed a feeling of worthlessness. Every time she looked at herself in the mirror, she saw an ugly little face looking back at her. She didn't see her big eyes, she saw her chubby cheeks. She didn't see her long, flowing hair, she saw her fat arms. She didn't like what she saw at all, and decided to do something about it. 

She stopped eating because she was sad, and saw that she lost weight. She liked herself better thin, so she continued starving herself, until she became an anorexic. Or, she felt better after she ate a whole lot of cake and ice-cream, but then she would vomit it all out because she didn't want to become fat. She became bulimic. And one day, when no one was looking, the little girl died. 

Sadly, this is not a story. The earliest documented case of anorexia is that of a 4-year-old girl, and about 15% of anorexia cases are fatal. While anorexia is currently more prevalent in the West, our society is fast leaning towards becoming more and more body conscious, and it won't be long before our children follow the same path. So treat your overweight child cautiously and whatever you do, don't damage her self-esteem. What's important is that you love your child - fat or thin. And more importantly, make sure your child knows it.