@MrItty It was not my child, so I cannot say for sure that he would have never been told; however, it was never a secret. The whole family knew about it, and it was understood by all that he would eventually have questions. The parents had hoped to tell him together, but he happened to ask his mother while they were at the hospital, so she told him then and there.
Another friend was conceived in a very similar situation. Many siblings were born after him. When he was five years old, he came across a copy of his birth certificate. He could read enough to know what the document was and to know the name in the “Father” section was not his dad’s name. As he grew up and noticed that his brothers did not look like him, he asked the question. His mother was trying to hide her past, so she lied to him, thus causing him to investigate many years later. Again, he was not bothered by the situation, just curious about the truth of his lineage. His dad was as much of a father as he could have hoped for.
True, both of these individuals had a reason to ask, but then again, what adoptee, etc. doesn’t have a reason to ask? Everybody knows if they look like their parents/siblings or not. There are always clues. As long as it is not hidden, there shouldn’t be a problem waiting for the child to ask the question.