That is an interesting tangent.
I take my children to mass, and its not optional. But I've never considered it to be indoctrination. They are free to fall away as adults, but nit before then.
Perhaps the missing element there is the negative connotation I attach to the word. If I teach my child that 2+2=4, and I do not accept another answer, is that indoctrination, or simply teaching?
I would view religion in that context. But I can see where someone would point to the "not optional" part.
When I discuss indoctrination in schools, my first thought is them not teaching facts, but enforcing opinions on students.
So someones understanding of the words used can change their meaning, making the entire discussion meaningless! Way to go!