Since various definitions of the term "religion" exists, we will probably not agree on it
- and your mentioning of the tooth fairy suggests a biased view on the term
I will however quote the first sentence on Wikipedia:
"A religion is a set of beliefs and practices, often centered upon specific supernatural and moral claims about reality, the cosmos, and human nature, and often codified as prayer, ritual, and religious law." http://en.wikipedia.org/wiki/Religion
A religion does not have to have a deity.
Therefore believing that god(s) does not exist is in my opinion as much a religion as e.g. Buddhism.
But I do agree that most people do think of atheism as a non-religion.