Another Scripture passage to consider is Matthew 18:10:

“See that you do not despise one of these little ones, for I say to you, that their angels in heaven continually behold the face of My Father who is in heaven.” (NASV)

D. A. Carson, Professor of New Testament at Trinity Evangelical Divinity School, had an interesting interpretation of Matthew 18:10:

“And if ministering angels are sent to help believers, what are the angels in Matthew 18:10 doing around the divine throne, instead of guarding those people to whom they are assigned? [. . .] The most likely explanation is the one Warfield (1:253-66) defends. The ‘angels’ of the ‘little ones’ are their spirits after death, and they always see the heavenly Father’s face. Do not despise these little ones, Jesus says, for their destiny is the unshielded glory of the Father’s presence. The present tense (they ‘always see’) raises no difficulty because Jesus is dealing with a class, not individuals. The same interpretation admirably suits Acts 12:15: what the assembled group thinks is standing outside is Peter’s ‘spirit’ (angel), which accounts for Rhoda’s recognition of his voice. But can the word ‘angel’ be pressed into this interpretation? Certainly Jesus teaches that God’s people in the Resurrection ‘will be like the angels in heaven’ as to marriage (22:30) and immortality (Luke 20:36).”
(From “Matthew,” The Expositor’s Bible Commentary, ed. Frank E. Gaebelein, vol. 8, page 401)

Of course, all infants are not elect, because one cannot change from an elect infant into a non-elect teenager. One’s elect status does not change. Still, I think one can make a strong case that all infants dying in infancy are elect. Three key passages make the point that the non-elect are judged for their own deeds, not the deed of Adam:

Revelation 20:13 – “And the sea gave up the dead which were in it, and death and Hades gave up the dead which were in them; and they were judged, every one of them according to their deeds.” (NASV)

2 Corinthians 5:10 – “For we must all appear before the judgment seat of Christ, that each one may be recompensed for his deeds in the body, according to what he has done, whether good or bad.” (NASV)

Matthew 16:27 – “For the Son of Man is going to come in the glory of His Father with His angels; and WILL THEN RECOMPENSE EVERY MAN ACCORDING TO HIS DEEDS.” (NASV)

Obviously, infants dying in infancy cannot act in a morally significant way that would lead to their condemnation. Arthur Pink commented, “An idiot or an infant is not personally responsible to God, because lacking in natural ability. But the normal man who is endowed with rationality, who is gifted with a conscience that is capable of distinguishing between right and wrong, who is able to weigh eternal issues IS a responsible being, and it is because he does possess these very faculties that he will yet have to ‘give account of himself to God’ (Rom. 14:12).” (From The Sovereignty of God, page 154)