Anabolism refers to the body being in a state of growth. If you want to grow as a child, or build muscle as a child/adult, you will need to eat an excess of calories and protein to grow.
Eat more calories. Even +1 calorie per day over maintenance will increase your weight. +3500 calories corresponds to roughly 1 pound of weight gain. Do +500 per day is 3500 or a pound per week, a very rapid gaining schedule.
3500 calories corresponds to 1 lound of fat, so to gain 1 lb of weight in a week, eat 500 extra calories per day. This is an ambitious rate of change, so anywhere from 100-500 extra per day will move the needle. Less than that gets close to the margin of error for calorie measurements.
Eat more protein. On top of eating enough of the needed macros and micros, eat an excessive amount of protein, especially if you’re doing the opposite of excess calories and eating insufficient calories to maintain weight. It is possible, but hard, to lose weight and gain muscle at the same time.