Cool mist. Per consumer reports, "Avoid using warm-mist humidifiers, or vaporizers, in children's rooms.
The hot steam can easily cause burns, and some of the surfaces might get
I found that there were no reasonably priced humidifiers with really good reviews out there. There are some really cute ones (shaped like a frog or something), but they don't have the best reviews. I ended up with the Crane Drop Shape Cool Mist Humidifier, which also doesn't have the best reviews, but seemed to me like the best compromise between a sort-of reasonable price, decent product, and decent reviews.