Adopted children under age 18 should be allowed to legally seek their birth parents without the permission of the adoptive parents. It's human nature to want to know where we came from and who we look like. There should be no law against trying to find out, regardless of the loving and nurturing upbringing the adoptive parents may have provided.
Even if the adopted parents are hoping to protect their children from possible issues with biological parents, the children still deserve the right to know about their roots. Prohibiting a child from seeking contact with biological parents is ineffective because it simply delays the inevitable. If the child has a desire to contact the biological parents, they will do so eventually. It would be better if the contact was done in an honest, cooperative fashion.
If an adopted child is over the age of 18 and is considered a legal adult, he or she should have the right to seek out his or her biological parents without consent. However, if the adopted child is underage, the adoptive parents have the right to keep that information from them.
Adopted children should not have the legal right to seek out biological parents until they are 18 years of age. Young children often do not understand why they were adopted in the first place, and attempting contact with biological parents might potentially lead to emotional trauma, confusion and dangerous situations. Adoptive parents are more mature and knowledgeable to make the most responsible decision.