Chest
Measure across the widest part of the Chest and shoulder blades (back), so the measuring tape goes completely around the body. For young girls this can be one inch below the bottom of the dress arm hole (underarm seam).
Shoulder to Hem, or Dress Length
Measure from the highest part of the shoulder (or the hollow in the throat) to the bottom of the hem (dress), over the fullest part of the chest.