When the apostles started out, they knew they had work to do. The whole world needed conversion. Everyone was pagan. That is, the world looked very much like it does today. The apostolic approach to the problem differed from ours.
Peter, for instance, did not set up a single parochial school. Luke did not write a children’s gospel. Not one of Paul’s epistles were decorated with yellow duckies. In short, according to the Scriptures and Church history, the apostles didn’t bother teaching children the Faith. They taught only the adults. Why?
Because the apostles understood the principle of subsidiarity. Pope Pius XI in his 1931 encyclicalQuadragesimo Anno described the principle succinctly: “Just as it is gravely wrong to take from individuals what they can accomplish by their own initiative and industry and give it to the community, so also it is an injustice and at the same time a grave evil and disturbance of right order to assign to a greater and higher association what lesser and subordinate organizations can do. For every social activity ought of its very nature to furnish help to the members of the body social, and never destroy and absorb them.”
The apostles knew they could not replace parents. Through the sacrament of marriage, God endows parents with the ability to teach their own children about Him. The apostles only needed to teach the parents the Faith, it was the parents’ responsibility to teach their own children. So, what has changed in the last two millenia? The answer to that is simple. Nothing.