Children going to jannah witout Being tested

Assalam alaykum My question is is allah not supposed to be just if children die before reaching puberty they go straight to jannah without any test and allah wont test them in jugment day either like the person who never jeard islam allah will test him in jugment day but he wont test they children that passet away in this world its like this children are doping an exam and they are bribing allah with there young age to go to jannah and Evert other person is tested if he pass he goes to jannah if he fails he goes to hell its like an exemple 3 students one studies his exam and becomes a doctor one didint study and didint become a doctor and one bribed his way trough the exam and became a doktor this is like the children who die before reaching puberty does this not make allah an unjust god he gives jannah free to some people and some people have to work there way to jannah