I agree that 12 year olds don't need to be going around by themselves, but believe me, an adult being with a group of 14 year olds is not going to stop them from acting crazy. I used to ride a bus with middle schoolers, and the driver tried best as she could to get them under control, but couldn't. I think it's more of a safety issue than anything. Most of the time, when kids get to high school, they learn how to conduct themselves around adults in a respectful manner, unless they are cheerleaders or Brazilians, that is. I took my younger brother to MK super bowl night, and believe me, we did not act crazy or in any way try to disrupt others in any way. I've taken him to Epcot several times as well without issue.
Next time that happens, ask the kid to stop. I can't believe that a Cast Member wouldn't do anything about that. It's disturbing another guest in a physical manner, which is unacceptable. There were some Argentinian girls trying to cut in line for Cyber Space Mountain last time I was there. A few words in Spanish from me and an adult stopped that fairly quickly.