I have a 6.5 yo, skinny and tall kid.  I ordered a Small and a Medium.

The Small one is just right length wise (suspected, thats why I ordered a Medium as a backup so he could wear it next year as well) but huge in the shoulders!  A picture showing that the shoulders in the Small are exactly the same width as in the Medium is attached.  Another picture is showing the proportions of the waist to the shoulder.  If my 6yo was a body builder than maybe.  The sleeves themselves are fine length wise, although pretty wide and bulky, but the ginormous shoulders make them way too long.  So the Small is going back.

Now, the Medium.  This one is a good 2 inches longer, which is expected and fine for us.  The shoulders are still too wide but hey, its meant for a kid 1-2 years older than mine (and therefore larger) so thats expected as well.  The waist is still super skinny.  But the sleeves?  Now those are another 2 longer EACH than already super long ones in the Small one. I don't have a 10 yo so I cant tell for sure, but I don't think humans grow arms that long.  Sadly, this one is also going back.

The fabric is fairly soft.  I guess I would prefer to have higher ratio of cotton to poly, perhaps 80-20, but the 50-50 would do if not for the super strange fit.

All in all, those are good sweatshirts for the money, as long as your kid has a body of a professional body builder with tentacle-like arms.