I totally understand that it hurts, but kids literally don't understand how their actions could affect people. They're noticing that something is different about how you speak and trying to process that. You can just use it as a teaching moment for the kid and anyone else who happens to be listening- or maybe ask one of your close family members to step in and say something before you get there. Our daughter was imitating her dad's stutter and I told her "that's called stuttering, people like Dad who stutter talk differently, and sometimes it's hard for him to talk. When someone is stuttering, the best thing we can do is be patient and listen." It really stuck with her!