You and I can speculate about what and how things happen in heaven. It will only be conjecture as every detail about what will happen has not been given in the Quran about this.
Also, the Quran speaks about 2 heavens, the one on earth by following Divine principles, and the one in the hereafter which is mainly referred to in symbolic, metaphorical language.
You and I need to concentrate and help develop the heaven on earth and leave the hereafter to the One Who will decide our affairs, including child deaths etc. No injustice will be done:
3:25 How will they feel when We gather them all, on the Day about the advent of which there is no doubt? Then, every person will be paid in full for what he or she has earned. No injustice will be done to anyone.