Fatherlessness

I have two children adopted from foster care. They are not biologically related. My daughter's birth mother never identified a father. My son briefly had visitation with both of his parents until parental rights were terminated. I have photos of my daughter's birth mother and my son's birth parents in their respective scrap books. Yesterday for some reason out of the blue my daughter, age 6, started crying and saying it wasn't fair that my son, age 3, had a picture of his daddy but she didn't have a picture of hers. She was really, really upset about it. She even drew a picture of a man holding a little girl's hand with a caption underneath: "I love you, Daddy. Please come home." For context, I have never been married so the kids don't have an adoptive father or even a father figure, really. My father is deceased and I'm not close to any surviving male relatives. The adoptions are closed so there will be no further contact with birth families, who the kids haven't seen since they were infants. There is no kid-friendly way to tell my first-grade daughter we don't have a picture of her birth father because her birth mother was a promiscuous drug addict and probably doesn't know who he was. I have been buying time by telling my daughter she has a father but he's lost (which is true, after all) and we weren't able to get a picture of him before he got lost. But at some point that's not going to be enough. Advice?