If you don't care for the often scented, and even more often expensive window cleaners on the market, there's no reason at all that you can't make your own cleaner at home!
It's as simple as mixing 1/2 cup each of ammonia, water and rubbing alcohol. That's it! Mix and put in a spray bottle, and you're ready to go. Spritz your window and wipe it down with a lint free cloth. The solution is also good for car windshields, mirrors, counter tops and other surfaces.
However, you should be aware that ammonia has been known to damage painted and tinted surfaces. It is not considered safe for eyeglasses that have a tinted coating, windshields with the same, or even specially treated windows. For those, plain soap and water is your best bet, or else a soap/water/vinegar solution.
In the old days, people cleaned their windows with water and vinegar, then polished them with old newspapers. You can still do this today, but vinegar will not cut oil or grease that may have gotten on the window, particularly in the kitchen. So if you want to get nostalgic and try this method, mix 1/4 cup of white vinegar with two cups water, and add 1/2 teaspoon of liquid dish soap, and put it in a spray bottle.
Be careful with the newspaper polishing, though. Given that many are now made with recycled papers, and eco-friendly inks, the results may not be quite what you anticipate.
Another precaution to teach children, is sanitation. While there are few disorders/diseases that can be passed from a fish tank to humans, the water does contain bacteria not found in the child's normal environment, so one of their first fish-keeping lessons, should be to always wash their hands after helping to clean the tank, or feed the fish.