In the hypothetical dialogue the youngest brother asks why he died early. Had he lived, he says, he might have believed and obeyed, thereby receiving reward like the first brother.
The objection appeals to a possible future. “I could have obeyed” describes one imaginable course; it does not establish that the course would certainly have occurred. Possibility and actuality must be distinguished.
The challenge depends on granting the disputed aṣlaḥ principle. If God must invariably bring about the most beneficial possible outcome for each individual, an explanation is demanded for the child's early death. If that obligation is not established, the argument changes in force.