Your estate plan has language that automatically includes all children naturally born or legally adopted as equals. Additionally, most states have also implemented inheritance legislation that puts biological and adopted children on equal footing.
However, there are a variety of situations that can complicate the issue. For example, adoption can sever the connection between the child and their birth parents in the state's eyes. So, the adopted child would not inherit anything from their birth parents' estate unless they were explicitly included in the Will or Trust by name. Additionally, unadopted step-children frequently do not have the same rights to inheritance as biological children.