It is important that you introduce the fact as soon as possible,I have seen terrible problems when the situation was hidden, and the child found out in their teens from the Birth Certificate, which of course says "By Adoption". I dont know your curcumstances, but the Adoption Agency or Local Authority you obtained your child from should be able to advise you. The "You were specially chosen" theme seems a good one.
Of course,knowing the rigorous conditions you had to meet, it was yourselves who are "special".