Best diet for lean girl aged 11 years?

Well balanced diet: Typically no need for a specific diet for a healthy pre teen child. If there are specific concerns, those would be best addressed with her physician, otherwise a well balanced meal plan will serve her best.

For weight gain?: Discuss with her doctor. Foods to help w weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocados, granola, trail mix, dried fruits, protein shakes & bars, lean meats, poultry, fish, smoothies (with added silken tofu or powdered milk), extra virgin olive oil, canola oil Greek yogurt (full –fat), pasta/ bagels & use of juice rather than soda.