After reading this thread, I hope that alot of the posters aren't parents! LOL!
seriously though, My son is 5. He's got his own 6 gallon Marineland tank, gaudy tacky, bright colors and stuff, but he loves it. He also loves to feed my 55's, and everytime he taps on the glass, I remind him of it. But, he still has to be reminded, because he's young, and doesn't think about it. Most of the time he's just excited and it trying to point at something to show me. However, he would never do it just to scare the fish.
An eight year old is still pretty far down in cognizant reasoning skills. This means that they will see a desired outcome, and be able to piece together the things that they need to create that outcome, but may not be able to understand possible other outcomes with those things. It sounds like the OP did the right thing, and it sounds like it is being rectified in an appropriate way.