Not accornding to Mormon doctrine. The child is NOT a sinner. God, being a just and loving God, would not hold a child accountible to complete things that she/her cannot possibly do. Whether it be sickness, death, mental retardation, etc, a child who is not capable of sinning cannot be held accountable for sinning. That is the word of God.
In other words, little children who die before the age of accountiblity, are alive in Christ and are saved.
As mentioned by an ancient prophet by the name of Moroni, sharing the words of his father, said:
"8 Listen to the words of Christ, your Redeemer, your Lord and your God. Behold, I came into the world not to call the righteous but sinners to repentance; the whole need no physician, but they that are sick; wherefore, little children are whole, for they are not capable of committing sin; wherefore the curse of Adam is taken from them in me, that it hath no power over them; and the law of circumcision is done away in me.
9 And after this manner did the Holy Ghost manifest the word of God unto me; wherefore, my beloved son, I know that it is solemn mockery before God, that ye should baptize little children.
10 Behold I say unto you that this thing shall ye teach—repentance and baptism unto those who are aaccountable and capable of committing sin; yea, teach parents that they must repent and be baptized, and humble themselves as their little bchildren, and they shall all be saved with their little children.
11 And their little children need no repentance, neither baptism. Behold, baptism is unto repentance to the fulfilling the commandments unto the bremission of sins.
12 But little children are alive in Christ, even from the foundation of the world; if not so, God is a partial God, and also a changeable God, and a respecter to persons; for how many little children have died without baptism!
13 Wherefore, if little children could not be saved without baptism, these must have gone to an endless hell.
14 Behold I say unto you, that he that supposeth that little children need baptism is in the gall of bitterness and in the bonds of iniquity; for he hath neither faith, hope, nor charity; wherefore, should he be cut off while in the thought, he must go down to hell.
15 For awful is the wickedness to suppose that God saveth one child because of baptism, and the other must perish because he hath no baptism.
16 Wo be unto them that shall pervert the ways of the Lord after this manner, for they shall perish except they repent. Behold, I speak with boldness, having authority from God; and I fear not what man can do; for perfect love casteth out all fear. "
Matthew, in the New testament said that one of the qualifications for baptism is to bring forth fruits meet for repentence. I child can't possiblity know who to bring for fruits meet for repentence as they are not capable of sinning. (See Matthew 3:8)