I agree with OM47, once every 24 hours, once a day is enough ammonia to be pumping into the system. Each 1ppm of ammonia results in about 2.7ppm of nitrite(NO2) and the factor going on to nitrate(NO3) is 3.6, with that final nitrate being acidic and driving the pH downward eventually, possibly slowing the process.
Sometimes it works (especially if the 7-year-old is crafty!) to involve them in the testing in various ways. Perhaps helping with a timer during the tests, perhaps graphing the results (graphs are easily tailored to different levels of difficulty.) Some kids can really get into drawing bacteria once you show them some pictures and that could even involve the really young one. Just dig into the nitrogen cycle, draw charts and be creative and for yourself, just keep remembering that explaining cycling is a lot more fun than explaining why a new pet has just died. Sounds like you're going ok though!
~~waterdrop~~