Could the Children Have Come From an Underground Civilization?

Could they have come from an underground civilization? It would be really cool if they had, and it might explain a number of things. There are no records of english missionaries having contact with an underground civilization and evangelizing them. With all the geological surveys and sismic mapping that we've done in the last century, any such civilization or its remains would have been found. So i don't think that the idea of the children came from an underground civilisation in england is plausible.