Thanks for your question.
The Bible makes clear that neither adults nor children go to heaven. When we die, whether adults or children, we go to the grave. When we are dead we know nothing. No part of us has a conscious existence after death.
Solomon with his God-given wisdom writes: Ecclesiastes 9:5,6 For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten. Also their love, and their hatred, and their envy, is now perished; neither have they any more a portion forever in anything that is done under the sun.
David, who had the Holy Spirit writes: Psalms 6:5 For in death there is no remembrance of thee: in the grave who shall give thee thanks?
The hope of the Gospel is resurrection from the dead to eternal life in the Kingdom of God (also called the kingdom of heaven). This is what is preached in the New Testament. 1 Corinthians 15:21,22 For since death came through a man, the resurrection of the dead comes also through a man. For as in Adam all die, so in Christ all(‘all’ that are ‘in Christ’ by belief and baptism) will be made alive.
The kingdom of God will be upon this earth. It will be established when Christ returns to the earth. At his return he will raise from the dead those that have been baptised into him. His kingdom will be the kingdom of David restored to bear rule over all the earth. Luke 1:32,33 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
This will be in fulfilment of the prophecy given through Daniel. Daniel 2:44 And in the days of these kings shall the God of heaven set up a kingdom, which shall never be destroyed: and the kingdom shall not be left to other people, but it shall break in pieces and consume all these kingdoms, and it shall stand for ever.
In death, those that have been baptised into Christ are said to ‘sleep’ awaiting his return.
So Paul explains: 1 Thessalonians 4:13-17 But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope. For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent (precede) them which are asleep. For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
Jesus said to his disciples after his resurrection: Mark 16:16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
If he had meant ‘Everyone shall be saved including children’ he would surely have said so. He clearly says ‘He that believeth and is baptized shall be saved’ and it seems reasonable to me that he expects us to believe what he says. Belief and Baptism (adult immersion) are both essential for salvation not optional extras. Salvation does not involve an ‘immortal soul’ going to heaven but rather the resurrection of a ‘dead soul’ from the sleep of death to eternal life in the kingdom of God on earth. This is what Jesus is referring to in the ‘sermon on the mount’ where he says: Matthew 5:5 Blessed are the meek: for they shall inherit the earth. If he had meant ‘heaven’ he would have said so.
Paul describes the reign of Christ on the earth: 1Corinthians 15:23-28 But every man in his own order: Christ the firstfruits; afterward they that are Christ’s at his coming. Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power. For he must reign, till he hath put all enemies under his feet. The last enemy that shall be destroyed is death. For he hath put all things under his feet. But when he saith all things are put under him, it is manifest that he is excepted, which did put all things under him. And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.
Jesus gives the following prophecy: Revelation 11:15 And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign forever and ever.
This is why the ‘Lord’s prayer’ has the clause: Matthew 6:10 Thy kingdom come. Thy will be done in earth, as it is in heaven.
The tragedy of this is that those who die before they are old enough to understand and be baptised will not be raised at the return of Christ. They will not be aware of their situation but their parents and relatives will bear the grief of their loss in this life. We need to view the situation as Job did in order to get the right perspective. Job 1:18-21 While he was still speaking, another also came and said, "Your sons and daughters were eating and drinking wine in their oldest brother’s house, and suddenly a great wind came from across the wilderness and struck the four corners of the house, and it fell on the young people, and they are dead; and I alone have escaped to tell you!" Then Job arose, tore his robe, and shaved his head; and he fell to the ground and worshiped. And he said: "Naked I came from my mother’s womb, And naked shall I return there. The LORD gave, and the LORD has taken away; Blessed be the name of the LORD."
If we see our children as a gift from God; we understand that it is God who both gives and takes away life and that he has the right to do as he pleases with both us and our children. We do not know why some children die, but we do believe that God has total control of both the good and the evil in our lives and works all for the eternal welfare of those who love him. Romans 8:28 And we know that in all things God works for the good of those who love him, who have been called according to his purpose.
As far as the kingdom of God is concerned, God has known from before the foundation of the world who will be in his kingdom. None will be lost, BUT the choice is always God’s. Romans 8:29 For those God foreknew he also predestined to be conformed to the likeness of his Son, that he might be the firstborn among many brothers. Ephesians 1:4,5 For he chose us in him before the creation of the world to be holy and blameless in his sight. In love he predestined us to be adopted as his sons through Jesus Christ, in accordance with his pleasure and will; Eph 1:11 In him we were also chosen, having been predestined according to the plan of him who works out everything in conformity with the purpose of his will. The best we can do for our children is to live Godly lives, teach them the truth and take comfort from what Paul says about them being (as a consequence of the way we live) ‘holy’ in the sight of God. 1Corinthians 7:14 For the unbelieving husband has been sanctified through his wife, and the unbelieving wife has been sanctified through her believing husband. Otherwise your children would be unclean, but as it is, they are holy.
May I suggest that you read After Death – What?
I hope this helps.