This is good that you have decided to raise this topic. Many parents have been stranded to what to decide. Well, I can say it is a personal decision. But when you look at it vividly I don’t think if it is. Like you have to keep that child into consideration. You have been raising that child like yours. So the decision you make should be wise. You should stop being self centered and value your child emotions too. What would s/he feel if she finds out you kept that away from them. Would you wish your child to be done the same? Do unto others what you would want to be done to you. I feel the child should know the truth. If their parents are still out there. Just tell them so that they know. That will not make you less of a parent. I understand many parents are afraid of doing this. Feeling like they will lose the child. But it is the right if the child to know their parents too.