I ordered the Little Girls size Medium for my daughter who is a petite 7/8 wearer. Holy moly this thing is paper thin. I bought it looking for a light sweater for her to wear in the areas of the school she says are cold, and boy is this sweater LIGHT. It also runs small. Like I said, she's petite, always has been, so it fits her but once school is up in 2 months I will probably pass it down to her baby sister who wears XS. If your kid is a full regular sized Medium 7/8, I would size up to be safe.