A 12-year old should burn less than 500 calories per day and take in 1600-1800 calories a day if the child has to stay healthy. The number of calories that are needed in the 12-year old will depend on his/her BMI and should be within the BMI range. However, if the 12-year old is on a weight-checking schedule, he/she should ask for a trainer's guidelines.
If you are worried you should ask a doctor. You should burn less than half of what you eat so you get the nutrients you need. Plus, unless your absolutely obese, you really shouldn't worry too much about calories.