Just got this shirt and my son were it this weekend it worked Great.  He is small for a almost 9 year year old and I got him the 8-9 shirt fits him great.  He would normal be a size 6-7.

Order two shirt size 6-7 for my 6 year old and size 8-9 for my 9 year old the 6-7 is small and the 8-9 is big.  The shirts look the same yet the size 8-9 is very  very long while the 6-7 is short  and barley covers my average size  6 year old yet if I got the 8-9 size for him it would be too long the 8-9 size is even big for my 9 year old.