While it is likely just kids being kids/not knowing any better, it is still wrong and I am sorry. As hard as it is someone needs to address the child. I work with special needs children 3 years old to 5th grade who are integrated into general education classrooms. As young as 3 peers can be taught to not tease or mock other students who are different. When they are young adults teach them to be kind. I am so sorry that there parents did not pull this child aside. Even though it would be tough if no other adults are willing to step up and properly teach a child, you should try. Explain to them everyone is different with strengths and weaknesses for you you have trouble speaking at times. But it is mean to keep pointing out the differences and ask how they would feel if someone did that to them. While I am saying this I have never been the one to redirect a child. Only twice has a child asked or pointed out my stutter (a niece and one younger child of a friend at a cookout). My mom quickly redirected my niece. The other child was around 6 and his sister who was 9/10 stepped in quickly as I stood in shock and said firmly "Logan stop! That is not nice and you cant talk sometime, so how would you feel and nothing in wrong with her (meaning me)" He quickly stopped and whispered sorry. That was it. I hungout with them many times for past 6 years and never mentioned again. I did tell myself if it happened again I would be ready to educate...least I hope.