Is he who collects donations for orphans, then buys some necessities for them with the money, and sends them the rest without informing them of the source of the money, sinful?

We advise you not to be attributed with a virtue you did not earn. It seems people think that all you give them comes entirely from you, and we fear that the Hadith: "The one who claims to have what he was not given is like one who wears two garments of falsehood" might apply to you. To avoid this, when paying them, say: "This money is not from me, or not all of it is from me; rather, it is from charitable donors," and ask them to pray for these donors. If you did not intend for them to perceive it that way, then there is no sin upon you, but what you have been guided to is better.