How to choose correct size

Use a new tape measurement (old one might be stretch and inaccurate) to measure your child. 
Chest: Measure around the nipple line with their arms down at their side. This measurement should not be loose or too tight. 
Height: Stand against a wall, measure from the top of their head to toe. 
Correct measurement is important to ensure a good fit. Compare their measurements to the chart "here" for sizing. If you ever in doubt, drop me a message. I am happy to help.