For a 12 year old, a healthy diet is very important as in this age the full growth and development of the mind and body starts taking place.Vegetables fruits and pulses should be main in the diet and the child should avoid fast food and cold drinks.
Here';s a sample diet plan-
Breakfast- Whole grain oats or Cereal, A glass of juice
Mid morning- Rava Idli/Cheese toast/Upma/Fruits
Lunch- Boiled Rice, Chapatti, Dal, Salad (You can write any healthy local dish)
Snacks- Raisins, Juice,Sprouts
Dinner- Soup/Salad/Chapatti/Chicken/Salad (Don't give a heavy meal. A light meal as dinner is good)
Avoid Any kinds of Junk food like bhel poori, samosa, pizzas, burgers etc. from outside stalls. Make it at home. Low fat food should be cooked.