It has not been widely adopted. Kids's T-Shirt Size Chart:
I've tried a few different brands and this isn't the worst but it's not the best. Personally I would recommend another brand. The best one I've personally tried and it's worked well for me and my friends you can find here: www.
Children’s sizes, US sizes. With the charts below, you’ll find out your child’s clothing size, in US sizes. The size is based on either the child’s weight and length in pounds and inches, or on specific measurements (chest, waist and hip). A majority of our shirts are on this blank. When somebody asks, "What size t-shirt do you wear?" the measurements on this probably match up to your answer. % . Please note that the measurements listed are an average taken from several different tees, so the t-shirt you order may vary slightly. If you find that you are in between sizes after taking your measurements, we always recommend ordering the larger size for the best fit.