About the default position. It seems impossible that babies can be born Christian, Baptist, Muslim or Mormon. How could that work? They could possibly be born ready to believe that something powerful and caring existed to protect and guide them. If so they would be correct. They are called parents. At some point in their development most children feel an urge to make their own decisions.
I completely agree, it's actually an evolved trait certainly in humans and in a large proportion of other animals for young to intrinsically trust their parents because it's beneficial for survival to lean on their experience until such time as they are ready to go it alone. A side effect of this trait is that children will tend to trust any authority figure they come in contact with, so when sent to Sunday School, for example, it is very easy for the messages to be transmitted as fact and ingrained at an early age and this becomes deeply seated in the mind of the child.
As for the original question, I don't ever remember believing that god did exist and I'm not sure why I ever would have. I'd ask when did you decide that you didn't believe that Spot the Dog existed, or the Billy Goats Gruff, or Cinderella? For me, the stories I heard from the bible or other affiliated publications were just that, stories. I really don't understand why any child would ever take them as anything more plausible than that unless they were taught as historical fact, which, regardless of my personal beliefs in the truth of them, I don't agree they should be taught in that manner.