i no longer have small children in my home.. but when my grandsons came to visit, Michael, the oldest was 3 years old. he was, and still is, fascinated by fish. which is good, because he's going to inherit my arowanas.. lol.
we explained to him that he couldn't hit the glass to get the fish' attention, not that that is ever a problem, cichla pay more attention to what is going on outside the tank than they do the inside. but he could watch at feeding time and he could help clean the glass. we made an extra spray bottle of vinegar and water and gave him a flour sack towel and showed him how to spray, then wipe. my aquariums were VERY clean when the kids left 5 days later. lol. we also took the whole family to the Aquarium of the Pacific in Long Beach while they were here.
Michael is still interested in fish, we went to the Georgia Aquarium together 2 years ago, when Michael was 6. we took Robert (#2 grandson) and dad too, but they were only along for the ride. lol. Actually, we all had a blast.