Jannah After Childhood Death
It can be argued that children that were going to die young were always destined to be part of Jannah, they briefly came to earth just to be a test for some, played their role and went on to become part of Jannah as expected. They may never supposed to be tested, just expected to play a part, just like the child that dies in the womb. Now it is indeed a loss if a person who enters the age of maturity and hence begins their test, only to die or be murdered at a young age, in which case, Allah will compensate accordingly. In case of the two sons of Adam (AS), the one who got murdered commented how his sins will be taken away from him and given to his murderer, hence achieving success.