Disbelife in Santa Claus, trolls under bridges and the Tooth Fairy is not generally considered religions.
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.