JOEL - This is a tough question and I am not sure anybody really knows the answer to it yet. First of all lets look at the scriptures that tell about the first ressurrection:
"And the graves were opened; and many bodies of the saints which slept arose,
And came out of the graves after his resurrection, and went into the holy city, and appeared unto many."(Mat 27:52-53)

"And now, the resurrection of all the prophets, and all those that have believed in their words, or all those that have kept the commandments of God, shall come forth in the first resurrection; therefore, they are the first resurrection.
They are raised to dwell with God who has redeemed them; thus they have eternal life through Christ, who has broken the bands of death.
And these are those who have part in the first resurrection; and these are they that have died before Christ came, in their ignorance, not having salvation declared unto them. And thus the Lord bringeth about the restoration of these; and they have a part in the first resurrection, or have eternal life, being redeemed by the Lord.
And little children also have eternal life."(Mos 15:22-25)

"And many graves shall be opened, and shall yield up many of their dead; and many saints shall appear unto many."(Hel 14:25)

"Verily I say unto you, I commanded my servant Samuel, the Lamanite, that he should testify unto this people, that at the day that the Father should glorify his name in me that there were many saints who should arise from the dead, and should appear unto many, and should minister unto them. And he said unto them: Was it not so?"(3 Neph 23:9)

Many have concluded from these and other scriptures that this very first resurrection would have included all the righteous adults and all children under the age of accountability who lived before Jesus. But since infant bodies have been found that date from before the time of Christ this conclusion can not be completely correct.
The Mosiah 15 scriptures say that all the prophets, all those who believed, and all who kept the commandments would be part of the first resurrection. None of these criteria would aply to infants or small children. Also, some of the scriptures above tell us that, at the time of Christ's resurrection, "many" of the saints arose; it doesn't say that all of them rose. The scriptures also tell us;

"but every man in his own order: Christ the firstfruits; afterward they that are Christ's at his coming" (1 Cor. 15:23).
"Behold, there is a time appointed that all shall come forth from the dead," (Alma 40:4).

This might explain why some infants who lived before Christ were not resurrected. Their appointed time has not come yet.
Actually the first resurrection is not over. Those innocent infants who are destined for the Celsetial Kingdom, can still come forth at their appointed time in the first resurrection when it continues at Christ's second coming.