When you are taking the neck measurement, leave 2 fingers between the tape and the skin. This will ensure that when the top button is fastened, that the collar is not too tight.
Measure around the fullest part of the chest, making sure that the tape is level under the arms and across the shoulder blades. Make sure that your child is relaxed when measured so that you get a true measurement.
Measure around the waist at the natural waistline (smallest part). In younger children this is usually midway between the bottom rib and hip bone.