Puritan Covenanter,
Since the Covenant of Grace is made with Christ, and through him, with all of the elect, (Larger Catechism 30 and 31.) One cannot simultaneously be a member of the Covenant and be lost. Some Reformed theologians talk about all of our baptized children being in the "shell" or the external administration of the Covenant, but only the elect are, strictly speaking, covenant children.
What jwithnell is articulating is the concept of presumptive regeneration, which is more emphasized by the Continental Reformed. We presume that since Christ is our God the God of our children after us that our children will be saved and ought to be treated as saved unless they show themselves to be Esaus. I do not believe that's what Enns is advocating here, for he explains his reasoning. It's not that we aren't to tell our children about the judgment of God upon sin because they're already saved and we don't want to confuse them. Rather he states that it's because they'll get the wrong view of God.... that the judg-y God is something that's reserved for more mature minds.
I entertained the presumptive regeneration view for a number of years, but have recently abandoned it in favor of presumptive election.... that we may presume that our children are elect and will be saved, but that does not negate the duty to evangelize them, for them to repent and profess faith in Christ for themselves, and for them to practice self examination and look for fruit.