The two dimensional planet is Flatland
The children see A Square's great great grandkids. the Black Thing is Nyarlathotep and IT is one of his avatars
Nyarlathotep’ true form is unknown, but has literally one thousand avatars, he is established as being one of the few, if not the only, outer god to be consciously evil. Mrs Who is related to a certain doctor
IT is [[Literature/It IT
]]