I think there is nothing bad in it to hide about. You should definitely tell your child later on. As the world is the small place and he/she will eventually find it out soon. So it is better that you tell them before they find out by someone else. The relationship is made up of love and care. So your children would want that more important than anything else. Also, ask your donor if she wants the child to know about her. As probably the child will get inquisitive about it and would want to know briefly. So it will be better to know about this in that way.