Of course, Children should have the right to know their biological parents. It is who they are. The child's genetics are from the biological parents. It is the child's choice however if they want to be part of their biological parents life who gave them up and need to understand that love is thicker than blood.
If offspring were allowed to find out who their biological parents, the emotional impact might be greater then imaginable if the parent is not adequate to be a good parent. Imagine if the parent were to be unable to be a good parent in the first place, a drug addict or someone who has gone to prison; it should be coherent that biological offspring should not find out about their real parents because it might break the already better situation they are living in. Biological parents also have a right if they want their biological offspring to try and contact them after a certain age. If the parent does not want to be disturbed for whatever reason, then that should be a given. One should not be forced to relive the bad situation that came upon the parent when they had to "give-up" their own offspring.