I believe this to be personal decision. Many people solve this case in different situations. Or rather there are different solutions to this question. It might seem so irrelevant. But at the end of the day it carries much weight. I know of various situation where kids have grown without knowing whether they are adapted. They come to know about it later in life. If the kid is fair enough, he or she forgives the parents. If not, then there will be ill feelings. Just know there is no secret that will last long. Especially if we are talking about adaption. In one way or the other the baby will later know. I understand how parents tend to protect this from their children. They feel if the children know of this then they will live them. I always say no to this. A child will leave you if the treatment is not good. The way you treat your adopted child leads to a lot of things than what you think.