At such a young age you dont want to imply to her that a diet is everything this is how some girls get eating disorders, also never tell her she is fat or any other harmful name instead imply to her that healthy weight provides a longer more fullfilling life. i suggest you get her in a activity she enjoys soccer, gymnastics, roller skating etc.. whatever it may be some sort of after school program, dont deprive her of the foods she likes instead teach her that eating healthy is the way to go (salads, low carbs etc..) still provide her with a treat maybe once a week kids still need to be kids. She could very well though have a medical issue that needs to be checked out sometimes that is the cause for weight gain and others it is just a sedatary life style, please see a doctor for further advice.
i hope this helped. do let me know!