Here's a reference which parallels the type of stuff that Thompson presents live and in his book, and apparently the reference to children spontaneously inventing the God concept is Petrovich, 2009, but this article doesn't drill down into what Petrovich is basing that statement upon. I may or may not search further in the context of this thread. (I've got a book to read by Tuesday which I haven't started, so no promises.)
Born believers: How Your Brain Creates God (from New Scientist) (I haven't fully read this yet, but as noted, I have other irons that need to be in the fire.)