Well, it depends on the gender of the 11 year old too. In case its a boy average should be lower than 2200 calories per day in order to lose weight.
And if it is a girl, average should be lower than 2000 calories per day in order to lose weight. However obviously the amount of exercise the 11 year old pulls in each day is a concern too.
For example a person having lesser calories and exercising as well will lose weight way easily compared to the other who is just cutting on calories.