Is it permissible for the primary individual who receives the social security pension on behalf of his orphaned siblings to take his own share of this pension without the knowledge of his siblings or their guardian, knowing that some of his siblings are minors?

If an individual's share from social security is known and fixed, there is no harm in taking it, and it is not a condition for the others or their guardian to know about it. For example: If the salary is 1000, and the individual's basic share is 500, he may take it without wronging anyone else.