The Children of Winter are a Druidic Tradition found in the Eldeen Reaches, primarily in the region known as the Gloaming. They believe that there is a natural cycle of life and death, that all things that are born will one day die, and it is through this cycle of death and regrowth that life ultimately flourishes. They see things like death, disease, and decay as part of this natural cycle, and thus they consider these things to be necessary for life to flourish.
Many of the Childen of Winter believe that civilisation has unbalanced this cycle of life and death, and many of them believe that if this cycle is bent too far from the natural order that Eberron will retaliate with a terrible fury. This theoretical retaliation is the metaphorical "Winter" that the Children have named themselves after, and some believe that this too is simply a part of the natural cycle of life and death. Many of the Children believe that the Mourning was the first symptom of this coming Winter. Many believe that saving those who cannot save or defend themselves is a subversion of the natural order, that those too weak to survive on their own shouldn't survive at all.
The Children of Winter despise undead, seeing them as a mockery of, and deliberate attempt to, subvert the natural cycle. Children of Winter will destroy any undead they encounter, and many of them dedicate a substantial amount of time to hunting the undead. Their primary location in the Gloaming is a Mabar manifest zone, and so the Children are often brought into conflict with the undead that the manifest zone spontaneously produces, and so the Children fill an important role in keeping the Gloaming contained.
There is a variety of beliefs within the Children of Winter, with some believing that the Winter has already begun and should be welcomed, that the only path to the next Summer is through the coming Winter. Others believe that disasters such as the Mourning are a warning, and that there is still time to avert the catastrophic Winter.