Is it permissible to offer an ongoing charity (ṣadaqah jāriyah) with the intention of returning unlawfully acquired financial assets to their rightful owners (the father and brother), without informing them of this matter? And what should someone do if they have unlawfully seized funds from an international company or a government department, given the difficulties in returning them directly?

You must return what you took from your father's money, whether you were an adult or not. The difference is in the sin, not in the restitution of rights. You must estimate the amount and return it to him unless your father forgives you. It is not a condition to inform him of this; it is sufficient to deliver the amount by any means. Charity does not suffice for it in this case. What you took from your brother's locker is your father's money, so you are obliged to return it to him.

As for what you took from the government entity, you must return it to them by any means, as well as the costs of the fictitious expenses for the company, even by transferring them to its account. If you are unable to do so, then spend it on public welfare and charitable causes. You can clear your conscience without returning to your homeland by using money transfers or by appointing someone to do so.