It is natural for any child, adopted or otherwise, to want to know where they fit and who they are. Finding one's identity is a natural part of development. Therefore, do not be upset, hurt or angry if your child is curious about their birth parent(s). Instead, be understanding.
Do not compete with the birth parents. Understandably, you are going to feel many emotions about your child's curiosity about their biological roots. You may react with feelings of resentment, sadness or anger. This may make you want to withhold information or speak negatively about the birth parents. Resist these urges.
From the very beginning, the birth parents should be talked about in a positive light, regardless of whether or not there were issues of substance abuse, psychological illness, abandonment, etc. Your child needs to feel that both you as their parents and their biological parents are good people.
It is important that parents separate their personal feelings about the birth parents. Parents must rise above their feelings in the interest of their child. It is extremely important to identity formation that children feel positive about their birth parents. From birth, children should be told as positive a story as possible about their biological parents. Be honest, but balance information with the child's developmental level. Parents should offer information on a "need to know" basis until children can incorporate a more detailed, full picture of their birth family circumstances when they are ready and interested in hearing it. As with many topics, it is important to wait for your children to express curiosity before telling them more than they may be ready to hear.
As parents, you know your child best. Try to sense when he or she may be developmentally ready to know the specific details about their biological parents, especially if there were issues of substance abuse, psychological illness, etc. Of course, levels of information should always be age-appropriate.