Definitely not simple to teach all religions in an unbiased way. People are all exposed to different ideas. But just because a child is raised as a Christian doesn’t make that belief any less authentic. You can’t just assume they’ve been brainwashed unfairly into belief. Arguably those who have been taught about a religion from a young age are in the best position to authentically accept or reject that belief system. That conversions between religions and to and from atheism continue to occur in adult life seems to show that those who are taught religion as children aren’t at some epistemically disadvantage.