In general I believe there are 7 reasons why we should believe that infants dying in infancy are elect and go to heaven.
1 - David appears to be comforted in the loss of his child not just resolved that it has happened.
2 - God says those who receive, interpret and reject his self-revelation in creation are without excuse, does this is some way not suggest that there is an excuse in some sense for those who never have opportunity to consciously receive, interpret and reject God's revelation?
Romans 1:20 “20 For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse,”
3 - judgment and condemnation is always described on the basis of the deeds done in the body whether good or evil not on the basis of the imputed guilt of Adam
This is not to suggest that imputed guilt is not important, (indeed I believe God must sovereignly act to overcome this guilt (as he does in all the elect)). Nevertheless these texts are representative.
2 Corinthians 5:10 “10 For we must all appear before the judgment seat of Christ, that each one may receive the things done in the body, according to what he has done, whether good or bad.”
1 Corinthians 6:9, 10 “9 Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, 10 nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God.”
Revelation 20:11, 12 “11 Then I saw a great white throne and Him who sat on it, from whose face the earth and the heaven fled away. And there was found no place for them. 12 And I saw the dead, small and great, standing before God, and books were opened. And another book was opened, which is the Book of Life. And the dead were judged according to their works, by the things which were written in the books.”
cf. also
Romans 9:10-13 “10 And not only this, but when Rebecca also had conceived by one man, even by our father Isaac 11 (for the children not yet being born, nor having done any good or evil, that the purpose of God according to election might stand, not of works but of Him who calls), 12 it was said to her, “The older shall serve the younger.” 13 As it is written, “Jacob I have loved, but Esau I have hated.””
James 4:17 “17 Therefore, to him who knows to do good and does not do it, to him it is sin.”
The next one is a bit philosophical but I'll include it anyway.
4 - The purpose of eternal punishment is that sinners will be consciously punished for their sins. An infant would not have been conscious of sinning in this life, could he therefore be conscious of the justice of God's punishment in the next life?
5 - It is perfectly possible for GOd to regenerate infants in the womb and thereafter by a sovereign act of grace, thereby recreating them apart from Adam's guilt.
6 -God represents the number of the redeemed as immense, innumerable, as the stars in the sky and the sand on the shore.....where are they all? Where have they all been? My belief is that those countless billions will be made up of those who were clearly Christians, and those infants and mentally incapable people throughout all ages.
7 - God is amazingly, bountifully gracious It must be stated, and stated without doubt that whatever level of GRACE we can expect of GOD, a more accurate estimate would be to double it, treble it, muliply by a thousand.
Psalms 86:15 “15 But You, O Lord, are a God full of compassion, and gracious, Longsuffering and abundant in mercy and truth.”
Deuteronomy 29:29 “29 “The secret things belong to the LORD our God, but those things which are revealed belong to us and to our children forever, that we may do all the words of this law.”
I believe based on the things God has revealed about himself, his ways, his judgment, his power, his mercy and his grace, that we can and should express an opinion on this subject, and my opinion is that of Loraine Boettner
"Where the Scriptures are silent, the Confession, too, preserves silence. Our outstanding theologians, however, mindful of the fact that God’s “tender mercies are over all His works,” and depending on His mercy widened as broadly as possible, have entertained a charitable hope that since these infants have never committed any actual sin themselves, their inherited sin would be pardoned and they would be saved on wholly evangelical principles."
And Warfield,
"Their destiny is determined irrespective of their choice, by an unconditional decree of God, suspended for its execution on no act of their own; and their salvation is wrought by an unconditional application of the grace of Christ to their souls, through the immediate and irresistible operation of the Holy Spirit prior to and apart from any action of their own proper wills . . . And if death in infancy does depend on God's providence, it is assuredly God in His providence who selects this vast multitude to be made participants of His unconditional salvation . . . This is but to say that they are unconditionally predestinated to salvation from the foundation of the world."
And Calvin,
"I everywhere teach that no one can be justly condemned and perish except on account of actual sin;"