Is it permissible to dispose of the orphans' money designated for them monthly to meet personal needs, while committing to giving them their due?

These funds belong to the orphans, and you are their guardian and trustee. You do not have the right to dispose of them except with the permission of the depositor. If the amount cannot be distributed in one go, then it should be distributed in installments without any transgression or unlawful disposition on your part.