Children of the Earth is the third chapter of the Youkai (Dissolving) Series.
On a kindergarten trip to a mountain, the children suddenly disappear. The parents search and eventually find the children in a deeply forested area, buried in mud. The children have become blank-eyed and lifeless. When the parents try to remove them, the children can't be pulled out of the earth - their bodies now go all the way down. After this, the children slip into the holes underground, and are never seen again.
It is never discovered what's happened to the children. Some people on the internet have claimed that the children simply returned to the earth, where all humans come from; but there is a public backlash against these claims.