I don't think you can designate what the money is spent for. From what I understand, an adult at the local project will help them spend it on something they need or will be useful. For example, when we sent one of our family gifts, she and her family were able to buy a parcel of land, which I had no idea would even be possible, and didn't know that was most useful/helpful to them.
I think I've always gotten a letter in response to a monetary gift, outlining what the child bought (sometimes including the amount spent on each thing, which didn't mean much since I am not familiar with the local currency), but no official receipt.