Humidity is always the wildcard. Different areas of the world require different percentages, so its not possible to tell you the best number. Sadly, we usually have to learn this by trial and error, and I'm sorry that your kids didn't have the best first experience with that trial and error phase.
55% is almost always too humid. It works for some, but I'd say a very small percentage can manage with 55%. My best recommendation is if you live in a humid climate, start with your incubator totally dry. It will still likely run around 25-35% without any water at all added. If you live in a very dry climate, start with just a little water, or even just a damp sponge.
Then on day 7, candle and check your air cells. Compare to one of the many charts that can be found online. If the air cells are too large, then add a little water. Humidity goes by surface area of the water, not the actual amount/depth. So keep that in mind also.
Its much easier to start low and increase the humidity to slow the air cell growth later, than vice versa.
** And just in case you are at a higher elevation, that throws a whole nother wrench in the mix!