Thank you for the replies, guys.
I noticed that there is emphasis on forcing, brainwashing, done over one's will and stuff like that. Regardless to definitions, isn't everything taught to children done like that? I see now that indoctrination works with all kinds of teachings, not just religious, and I ask the above question having that in mind now, not just for religious.
We can't really ask a 4-5 year old if they want to go to school now, can we? Or if they should go to the doctor if they are sick, to sleep early to wake up early for school, eat specific kinds of foods, respect what and who should be respected (socially for now), etc. etc. Parents would, or I dare say rather should, actually push such things to their children whither they or you like it or not. Dunno, but as for me, I wouldn't leave a 4-5 year old do what they would randomly with a claim that says "hey, they should do or believe what they want".
Having the above said, I noticed that "indoctrination" is used extensively with religion and as a negative connotation. If it is negative or wrong, why would it be so but deciding for a 4-5 year old to go to school is not? I think that would be a double standard. There is no such a thing a brainwashing children with religion. As long as the children are not mature enough to decide for themselves, their guardians have full right to teaching them anything they see right (again, I'm talking within standards, not with exceptional bad cases).
So, why it is okay with non religious teachings and wrong with religious? (Which I believe is a double standard).