If you already have children and have decided to adopt another, all of the experts I know of agree that it is imperative that you let them in on your plan immediately. This is a decision that affects all aspects of their lives as well, and they need to get used to the idea.
Responses vary; some kids are happy, others may display jealousy. You want to know about any negative emotions your children may have so that you can point out the benefits and rewards to them, and change their attitudes. Of course, just brushing their concerns away will make the problem bigger. Instead you need to listen patiently while reassuring them that you understand.