Wait, hold up.
You want to know the most effective way to get brainwashed? Be born. That's about all it takes... the entire psychological development of a child revolves around very sophisticated brainwashing procedures as implemented by family and culture. We imprint infants and children with very specific information at very vulnerable points throughout their psychological development in order to (hopefully) make them conform to the acceptable design as mandated by the society in question, which often also includes religion.
All of you are brainwashed, me too, to a large extent. I've managed to recondition myself a fair amount but for the most part I'm still a product of the society!
Unfortunately for many of the contenders in this thread, I regret to inform you that atheism is as much a system of belief as it's religious counterparts. In many cases it conditions one to be entirely unreceptive to the information construed through religious and spiritual mediums which I feel has some very valuable potential if you can see through the bullshit of it all.
Sooo, with that in mind, I would say that the only fair approach is that of an informed agnosticism towards children's spiritual and religious development. No belifes should be forced but a wide array of information should remain available, which means teaching them about a variety of religions (and lack of religions) so that they have the whole buffet in front of them to analyze so they may make the most informed choice.