We use humidifiers a lot at my house. I would suggest an ultrasonic type of humidifier. They are almost silent and put out a very gentle mist. We have mostly used the drugstore brands, Vicks, Homedics etc. They all do the trick. We were told that the cool mist feature is an important one for children. The mist from the warm ones can burn little ones. We have been through a ton as we had them going a lot when my kids were little and they kept knocking them over and breaking them :). Keep the filter clean and you should be all set!
We have the Crane Droplet Ultrasonic Cool Mist humidifier. Had the cute animal ones when our kids were little but they didn’t seem to last too long. Finally decided to get a not as cute, but much more durable model. We have been happy with this humidifier.