Yes she was a child, but a child who was paid for doing a job, a professional job that she could have easily walked away from if she had really needed to relieve her "Boredom".
The reason that it is wrong to play pranks is that individuals can be harmed in the process. If the prank causes scenery to break that can result in individuals getting hurt. Performers unknowingly being subjected to fumes that make them ill isn't funny at anytime under any circumstances and shouldn't have been tolerated...period! I don't think that Andrea's behavior would have been okay had she done that same thing in a classroom or on the subway do you? I also doubt that you would laugh off her behavior had one of the Orphans been your child.
It seems like the lack of punishment she received in her ANNIE days has resulted in her thinking she can do whatever she likes whenever she likes. And by now she should have figured out (all on her own) that such things are completely inappropriate no matter what age you are.