Bobby - I think I would wait until its a "sure thing" or else word it with your kids that you might be taking care of "Little Guy" for a while and you are working on it. I would take that approach so that way they know if the social workers come around etc.
Also, the school age children will need to be interviewed, so its best they know its going on before then. You also might talk about it in generalities so they get used to the idea of adoption, new kid etc. Mom and Dad think we need a new brother or new sister in our family, what do you guys think? etc That way you are introducing the IDEA without a specific child being blamed for the disruption ... once they are used to the idea, introduce the idea of which kid it is
Jen