Should adopted children be allowed to contact with their birth parents?

Yes at some point in time. Eventually the child will find out about the adoption and it's not fair not to discuss the adoption when you feel it's the right time. Of course there are laws and some States won't give out the information as to whom the parents are. Some parents that have given their child up for adoption don't want to meet with their child for reasons of their own. If you know the mother, father or both then call them and discuss it and arrange a meeting with your child and their birth parent's.