For indoctrination of children, you need (1) a coherent system of doctrines, (2) a strong group identity of those holding the doctrines, and (3) customs or doctrines about the infusion of these from the youngest ages. The only significant non-religious belief systems I can think of which fit the bill are nationalisms and Marxist-Leninist communism – and indeed people do or did speak about 12-year-old, six-year-old or even newborn Germans and Americans or Communists, and indeed I resent both. Now, the first of the above is not incompatible with religion while the second is practically dead. In the 21st century so far, salad-bar-type theists get ever more numerous in the West but are still a small minority world-wide, while lack of ideological organisation is typical for the non-religious. Both may change in the future, but as things stand, you won't find many equivalents of Sunday schools, prayers, crucifixes on the wall, symbolic cloth items or the fear of the devil among six-year-old children of non-religious parents. A further issue you don't seem to be taking into account is that a lot of personal views expressed here aren't inspired by the viewpoint of an atheist teacher but the viewpoint of a one-time child who suffered through the 'teaching' of theist parents or school teachers.

Now, what is your opinion about the political parallel? Would you approve of Free Democrat schools, Sunday instruction of six-year-olds in the basic tenets of Social Democracy at the local party headquarters, and the same children wearing party insignia to school? Should voting age be eliminated? Why should parties be more constrained than religion?

_{*Lunatic*, n.
One whose delusions are out of fashion.}