What is the legal ruling on returning the orphans' money, which was lost in trade, to its owners as a gift without informing them of the loss, knowing that I stipulated bearing the loss on their behalf?

If you acted with the money of your orphaned siblings without being their guardian or without their guardian's permission, then you are responsible for their money and must return it; because your action is impermissible, and because the permission of minors is as good as non-existent.

However, if you were their legal guardian, or if the legal guardian permitted you, or if they were adults and you stipulated for them a guarantee of their capital in the Mudarabah (profit-sharing) with this relative, then there is no harm in that, provided that the stipulation is independent of the Mudarabah contract, and this is in accordance with what has been approved by the Islamic Fiqh Academy.