Please read more carefully, and explain how the invincibly ignorant in the state of grace can commit grievous sin to forfeit grace.
A baptized child of 6, who is in the state of grace, is NOT invincibly ignorant. Quite the contrary. A person in the state of grace, at the age of reason, has the Holy Ghost within him and thus is prompted by Him to a) stay in the state of grace/follow the 10 commandments and b) seek God and Truth (if they aren't practicing the Faith or don't know of it).
There are innumerable stories of young children of 7, 8, 9, 10 years old who were baptized as some type of protestant. These children had catholic friends and the protestant children would go to the catholic church a few times in their childhood with their friends. It wasn't until they became teenagers or young adults that they converted to the Faith because "they always liked the Catholic Church they had visited" and/or "there was something I was drawn to".
This is grace building upon good nature. They converted in God's time and followed His promptings.
Or how about the millions and millions of apparitions/holy dreams that are happening all over the Middle East right now? Muslims are hearing about Christ and He is appearing to them in dreams, telling them to "Follow Me" and then He gives them instructions to go to some place, at a specific time and look for a specific person. Who turns out to be a Christian. They end up getting baptized.
A muslim would be "invincibly ignorant" and look at what God is doing for them. Look how He is calling those of good will.
A baptized child is the Temple of the Holy Ghost...all God has to do is speak to them in their heart. There's no way they are ignorant (unless, by sin, they close their heart to Him).