You will have to measure the "head" yourself to get proper measurements.
You only need 1 measurment, I recomend the wight of the head, since it´s the easyest to take.
Simply put your kid's head between 2 books, and measure the space between them, then, add 2cm.
For example, if the head has 18cm wight, then put 20cm as helmet wight, the height and depth will be calculated automatically by pepakura.