You can use the noun unbelief to talk about a lack of belief in something, like your unbelief, since childhood, in fairies.
When someone doesn't believe in something, that person has an unbelief. Most often, the term unbelief is used to talk specifically about religion. An atheist is characterized by her unbelief in a god or higher power beyond things that can be scientifically proven. Another word for unbelief is disbelief.