To be honest, your poll choices weren’t very clear. But I think it would be a terrible thing to hide an adoption from a child, and yes, I do think that this would be a sin, because eventually the child will find out. The child will probably notice that he does not look like his parents or the other children, and eventually someone in the family will spill the beans. Even if no one in the family tells the child, and the child somehow looks so much like his adoptive parents that he never finds out, what happens if he has some medical problem, such as the need for a kidney transplant, and the parents have to tell him then that he was adopted?
Any, and every, child has the right to know the truth about where they come from. Even if a child never meets his biological parents, they are still a part of his life and his history. He deserves to know this so that he can fully understand who he is. If I worked at an adoption agency, and I heard about parents who were going to deceive their child, I would never approve the adoption.
Granted, I have never adopted a child, so I don’t have direct first hand knowledge of adoptions. But my cousin was adopted from Korea, and my mom has two friends who adopted children; one adopted two children from India, the other adopted a child from Russia. I was raised with adopted children in my family and friends, so to me it was as normal as having a biological child. So while I am not an adopted child, and I have never adopted a child, I’m not completely unaware of the realities of adoption, especially now that I’m older and my mother has shared more details with me.