I ordered the size I (6-8) for my 6 year old in white and sky blue. When it came, I'll admit they looked tiny and the cut of the leg holes looked high, but I had her try them on anyway, and I'm glad. They fit perfectly, and she says they're really comfortable (a rave, since she was complaining about her other leotards being itchy and/or bunchy last weekend). For reference, she's 42 lbs, 48 inches, and it appears she could probably gain a few pounds or inches and it would stretch to fit.
This leotard was at least two sizes smaller than what is listed on the size chart. I did measurements on my daughter and based on her measurements size Medium (8/10) would be the correct size. We received the leotard and it looked like it was for a girl who wears a size 5/6.