Is it a sin to not tell a child that they are adopted?
There are many times when we restrict access to information to only those who need it. A government does not tell everything about its security preparations because those against the government will use that information to find weaknesses. A salesman, negotiating a contract, will not reveal how desperately he or his company needs that contract to stay in business. To do so would be inviting the buyer to force lower prices. Not saying everything you know is not necessarily a lie, it is a mark of wisdom. "A prudent man conceals knowledge, but the heart of fools proclaims folly" (Proverbs 12:23).
If it is obvious from physical characteristics, then it is likely the fact that the child is adopted will be pointed out to the child. Thus, the question would be whether you want to explain the situation before this happens or wait until it becomes an issue. If the child asks, you cannot lie. In such a case, you need to explain why you chose him to be a part of your family.
If it is not obvious from physical characteristics, then you can delay telling the child if you so desire. A child needs to know he has a mom and dad who both love him and want him. That is the most important thing. If later in life he has genetic problems or needs to know his family's medical record, then you can explain that while you are his parents, you are not his birth parents. However, you have caused no harm by not mentioning something that might traumatize the child.