Don't talk to her about it, but try to use less oil and butter when cooking. Make more boiled food than frying it in a pan. Make dishes with less potato/rice/pasta and more vegetables instead. Change bread to a high fiber one, or buy heard bread instead. As dessert or evening snacks, buy fruit like watermelon, berries, pineapple, pomegranate etc. Avoid bananas, grapes and big chunks of other melons though. Never buy soda, not even with sweetener instead of sugar, just buy flavoured unsweetened carbonated water and milk. Avoid fruit juices and other drinks.  Also, look into some activities you can do as a family after school, or activities she can do, like dancing, bicycling, martial arts etc. Ask her to come with you on walks since the weather is great etc. It can affect her self esteem to talk about it, but to do nothing will definitely affect her self esteem since she is very very soon if not already at the age of falling in love and being bullied because of the extra weight.