Why are Little Children Excluded from the Grace of God?

[Book of Mormon]
And now, king Benjamin, thought it was expedient, after having finished speaking to the people, that he should take the names of all those who had entered into a covenant with God to keep his commandments. And it came to pass that here was not one soul, except it were little children, but who had entered into the covenant and had taken upon them the name of Christ.
Poor little children. Why had they not entered into this covenant and taken upon them the name of Christ. If we are talking about the same Jesus Christ of the Gospels Matthew, Mark, Luke, and John, it would seem odd that his name was not given to the little children, that they were not also saved by this “covenant.” The Jesus of the Gospels loves little children. In fact, God, in revealing his gracious will to the world through Holy Scripture, shows that he loves little Children when he sets up a “covenant” with Abraham.
I put the word covenant in parenthesis, because I think it is slightly misleading. God gives a testament through which his people inherit grace. As Paul talks about it in Galatians, and indeed elsewhere, he talks about people being heirs, coheirs, and inheriting. This language tells you what kind of “covenant” God has made with Abraham, and it is not your run of the mill 50/50 covenant, but a testament, a will. “For if it is the adherents of the law who are to be the heirs, faith is null and the promise is void.” (Romans 4:14 (ESV) “And if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him.” (Romans 8:17 (ESV) “ And if you are Christ's, then you are Abraham's offspring, heirs according to promise. (Galatians 3:29 (ESV) “This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.”( Ephes. 3:6 (ESV) “So that being justified by his grace we might become heirs according to the hope of eternal life.” (Titus 3:7 (ESV) You don’t become an heir by any old “covenant” but by a Testament. And God is not so fickle a father to cut little children out of the inheritance because they are too young to make false promises of obedience to Him. In fact he commands Abraham that all his children are to be circumcised on the eighth day. Because the circumcision was a seal of this testament he made with Abraham. And in the gospels he shows himself to be very loving towards these little children, taking them in his arms and promising that the kingdom of heaven belongs to such as these, in fact going so far as to say that if your don’t receive the kingdom of God as a little Child you will not receive it at all. “ But when Jesus saw it, he was indignant and said to them, "Let the children come to me; do not hinder them, for to such belongs the kingdom of God. [15] Truly, I say to you, whoever does not receive the kingdom of God like a child shall not enter it." (Mark 10:14-15 (ESV) Later he extends this kingdom to them promising that the gift of the Holy Spirit through baptism is not just for adults, but for their children also. “ And Peter said to them, "Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. [39] For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself."( Acts 2:38-39 (ESV)
So now why are they excluded from these promises, cut off from their inheritance etc. in the Book of Mormon? Perhaps because it is a work of fiction written by an ex Campbellite/Baptist preacher named Rigdon? Perhaps because it has nothing at all to do with the Jesus Christ of the New Testament? Just a couple thoughts.