You might want to go to this site:
http://www.allergybuyersclubshopping.com/warm-cool-mist-h...
They have tons of options, rankings, and articles about which humidifier is best. For me (and the site) the best is the air-o-swiss model 7135. You have the option of both warm and cool mist. (BTW my pediatrician says it doesn't matter the temp of the mist bc it will take on the temperature of the room instantly-- I found this to be true.)
It comes with a silver ionic stick which kills bacteria, mold and viruses, so you don't have to clean it everyday-- just once or once every other week. It also has a demineralization cartridge that prevents white dust that so many humidifiers can produce, which harms the airways and lungs. Another bonus is that it has a digital readout so it self-regulates the level of humidity in the room, so you'll never worry if it's too humid or not humid enough in your child's room.
It is super quiet and I've been really happy with it this past winter. I feel like my son is much more comfortable while he sleeps. (Yes there are models that are noisier than others.)
Depending on the size of your room, some models are better fit than others. The 7135 is good for 600 sq ft. I think the newer model covers more sq footage.
Good luck!