First of all, it is the position of some of your fellow religionists that that is the case. I don't believe in either heaven or hell, so it certainly isn't any position of mine.
Secondly, they make a good case. The Bible pretty plainly states that the way to heaven is through a belief in Jesus -- the only way, in fact. Those religionists who believe that other religions have no chance (including their children) of making it to heaven (and thus the alternative is hell) have a strong documentary case -- despite your latter day interpretations.
Again, I can only direct you to Romans Ch 1 & 2. Written circa 55 C.E. (or A.D. if you prefer, I know I do). Hardly a latter day interpretation. You have also been directed to Matthew's Gospel, and some how pulled out an interpretation that I have never heard expressed in any church. But Christ's meaning was pretty clear: "The Kingdom of Heaven is for such as these". These kids had not made a statement of faith, tithed, or worshipped in a church. Notice too, in the first book of the "Left Behind" books, that as part of the story line, every child in the world disappeared at the same time the Christians did. No serious christian theologian tore into the authors for that; they were criticized for other things in those books. We can take a discussion of the concept of the "Age of Accountability" to a private venue if you wish.