You have said that all children who die below the age of accountability go to heaven. Will all the children below the age of accountability who are present on earth at the time of Rapture be leaving along with adult believers irrespective of their parents belief or religion? If it is so, then won’t there be any children left on this earth for the Tribulation? And how shall God take them without the consent of their parents, since they may be of different religions other than Christian?
I believe the Bible teaches that no matter who their earthly parents are, all children belong to God until they are old enough to be accountable for themselves (Matt. 18:10). The Bible also teaches that they are not responsible for their sins during this time and have eternal life (Romans 7:9).
In my opinion, when the rapture comes all children will be taken. After all, Jesus said the kingdom belongs to such as these (Matt. 19:14) and unless we become like them we will never enter the kingdom (Matt. 18:3). Their unbelieving parents won’t know they’re gone until it’s too late because they will be here one moment and in a flash they will have disappeared (1 Cor. 15:51-52).
The world will be without children for at least the first nine months after the rapture. After that mothers will begin giving birth again as before. We know this because in Matt. 24:19 Jesus said the Great Tribulation will be a terrible place for pregnant women and nursing mothers.