Chest Width (flat, front) : Take a current shirt that fits your child. Lay the shirt flat and measure the front of the shirt, going left to right, from directly under the sleeves. This will give you the flat front chest width.
Chest Width (around, under arms) : Using a paper tape measure, take your child's measurement under the arms and around their chest, going fully around their body. Make sure tape measure is not too tight, you should be able to fit 1 to 2 fingers between tape measure and child's body for best fit.
Body Length at Back : Take a current shirt that fits your child. Lay the shirt flat and measure the back of the shirt, starting at the neck and ending at the bottom of the shirt. This will give you the body length at back measurement.