The temperature part of this question is tricky, because different children are comfortable with different temperatures, just like adults.
For example, each of our children have been different as far as getting too hot or too cold. We have 18mo twins, and we have to dress our boy more warmly than our daughter. My 3yo sweats considerably during the first part of the night, then he seems to cool off; so he needs to be uncovered at first, then covered. My 5yo will pretty much sleep with any temperature setting. My 15 yo is always warm, so he blasts the fan in his room.
In general, we try to keep our house at 71-72F(22Celcius) at night, and this seems like a good average temperature for everyone.
I'm not sure about humidity. We've never tried humidifiers for the kids, but we live in Houston which has a humid climate. With the high humidity, mold is definitely an issue in Houston in general. So long as you have painted walls in the room you're using the humidifier in, the areas to watch out for mold growth are typically near vent hood covers.