As someone else said, it is possible they did not know why or what it was. Explaining to them could help, but it is possible that for them it was first contact with something unknown. Sometimes your brain just decides you will laugh and you literally cannot help it. If you, however, do not want to try this again, you could really try the adults. They generally do know what is going on, and while it can be more intimidating, they definitely will not laugh (except if they are total assholes). Now I do not stutter myself, but these things apply for just basic public speaking too. Kids tend to be unruly and laugh about everything, while adults, well, are adults. If youd like some advice on how to speak in front of a serious audience, feel free to DM, as i have quite a lot of experience in this regard (im that person they (school or city council) ask to speak at most occasions by default, for better or worse, and have had to help my more socially anxious peers with this many times when it so happened that they had to stand out there.).