My First Mom Attack

So Colt and I were at the park yesterday just enjoying the day.  He loves to run after the bigger kids and try to do all the stuff they are doing (like climb the rock wall).  Yesterday he was running after a little boy who was about 6 or 7 and Colt tripped over himself and dropped his sippie cup of milk.  This is a very common occurrence in our house and I thought nothing of it.  He got back up, brushed himself off and just laughed.  Next thing I know the "big kid" turns around and starts pointing and laughing at Colt and said, and I quote, "hahaha, look at him, he is stupid!".  I was heartbroken.  Did someone just call Colt stupid?  I know the kid meant nothing by it and Colt didn't have a clue what he had said either but it just killed me.  Well....next thing I know I am grabbing the little boys jacket and explaining to him that "he is NOT stupid, he is ONE year old and just learned to walk."  I also made him apologize and told him that was a very mean thing to say to someone.  Whoops, I guess I shouldn't be disciplining someone elses kid but I just couldn't stand for someone calling Colt stupid!  What on earth will I do when he is in middle school!??! Lord help me!
Here are some crappy camera phone pics of Colt playing at the park: