When you travel from the earth’s equator to the North Pole, you pass through several climate zones: from tropical to temperate to arctic cold.
Yet, you can experience the same sort of climate zones when you climb a mountain in a tropical country, but you can do so in a shorter span of distance and time.
I see a parallel in the covenant history. The three stages took thousands of years, yet in one person’s life they can be experienced within a span of less than a century.
Although children in the church are fully part of the two-dimensional covenant, just like the children of Israel, yet they cannot make a credible life-long commitment to follow Jesus. We do not expect them to commit themselves to a life-long partner in marriage, so we cannot expect them to make a similar decision towards Christ, even though He has –in a sense- already offered them His wedding ring!